Build an AR-15: AR Calibers

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The AR platform can be adapted to dozens of different calibers, like this Rock River Arms LAR-6.8 chambered in 6.8 Remington SPC.

The AR platform can be adapted to dozens of different calibers, like this Rock River Arms LAR-6.8 chambered in 6.8 Remington SPC.

THE SERIES

In these troubled times where everyone wants to stir up trouble and hate over our differences in appearance, I want it to go on record that I don’t just tolerate diversity, I embrace it.

In my safe right this moment, I have AR-type rifles chambered in .223/5.56mm, 6.8 SPC, 300 Blackout, and .308 Winchester. Of course, .308 is admittedly related to the Armalite AR-10, not AR-15 family. When it comes to diversity, I put my money where my mouth is. As we speak, I’m Jonesing for something in 6.5mm. Not only that, I’ve been begging the folks at Smith & Wesson to come out with an M&P 338, chambered in, you guessed it, .338 Lapua. Why? Because diversity is fun.

One of the limitations for caliber swaps is the length of the standard magazine and magazine well.

One of the limitations for caliber swaps is the length of the standard magazine and magazine well.

The neatest and spiffiest thing about AR-type rifles is that they’re not really just a single rifle type. They’re a platform, or set of common design specifications on which you can build a whole lot of differrent, but related, guns.

The split receiver design of AR-type rifles lends itself to using a standard lower receiver with stock, grip, magazine well, and fire control system containing trigger, hammer, sear and safety mechanisms. Then, in theory, and for the most part practicality, you can mount different upper receivers that house the chamber, barrel, bolt carrier, bolt and hand guard. Of course, you can’t just put any caliber upper on a standard lower. The primary reason for this is that the magazine well is part of the lower receiver, and that creates some limitations on cartridge size. For example, one of the big differences between AR-15 and AR-10 type rifles is the larger magazine well on the AR-10 to accommodate .308 Winchester. Everything is bit larger owing to the need to handle bigger cartridges.

.223 Remington / 5.56x45mm

What’s not to love about the “original?” Well, more or less the original. Back in the ArmaLite days when the AR direct impingement rifle was being developed, the original plan was to make it a .308 / 7.62mm rifle, but the military folks liked the smaller caliber option. The thinking was that the average soldier could hump a lot more cartridges if they were smaller and lighter.

The debate over terminal effectiveness will outlast all of us, so I’ll stick to the theoretical performance idea. A very small and very light bullet, moving insanely fast, should fragment, tumble, and wreak all sorts of havoc on organic targets. There’s plenty of evidence that it does. There’s also plenty of evidence that it’s not a magic one-shot stopper like the old 7.62 and .30-06 ammo fired from M-14s and M1 Garands. More suppressive firepower weighed against better terminal effectiveness per hit? You make your call as to your personal preference.

One thing that has become interesting is the standard caliber AR platform’s suitability for home defense. That same theory that says a light and fast bullet will fragment and tumble also helps with potential over-penetration in the home. If you want to perform a fun science experiment, take a bunch of wallboard to the range, separate it to create artificial walls, and shoot it with 55 grain .223 ammo and some normal handgun hollow-points. You might be surprised at which one penetrates the most interior walls.

.300 AAC Blackout / .300 Whisper

I love this caliber in an AR platform. Before the hate mail starts, I’m not claiming it’s “better” than any other AR cartridge, I just really enjoy it, mainly because I love to reload ammunition. The 300 AAC Blackout allows for a near-infinite variety of bullet weight and velocity combinations.

The huge 300 AAC Blackout subsonic projectiles are stuffed into cut down .5.56mm cartridge cases.

The huge 300 AAC Blackout subsonic projectiles are stuffed into cut down .5.56mm cartridge cases.

You can launch a 245-grain hunk of pure lead downrange at 1,000 feet per second. This flying brick approach yields 544 foot-pounds of energy. Using a suppressor, this is a freakishly quiet combination sure to elicit very un-macho giggles from anyone present at the range.

You can also go supersonic and zing an 110-grain bullet at 2,500 feet per second. This .30 caliber projectile cranks out 1,527 foot-pounds of energy. That’s about 50% more kinetic energy than a standard .223/5.56mm 55-grain bullet moving at 3,000 feet per second. It’ll stay supersonic out past 600 yards or so depending on local conditions.

As an AR-type rifle cartridge, I love that it fits in standard .223 / 5.56mm magazines. That’s because it uses cut down cartridge cases from .223 / 5.56mm. I don’t mean vaguely similar, I mean it’s literally the exact same case, trimmed down and reshaped with a lower and less aggressive shoulder to accommodate a .30 caliber bullet. There are some 300 Blackout specific magazines that do a better job of handling odd-shaped bullets, but, for the most part, compatibility is excellent. It even uses the same bolt carrier and bolt as the 5.56mm.

Use of shorter barrel ARs prompted development of alternate calibers like this 300 Blackout to gain improved terminal performance.

Use of shorter barrel ARs prompted development of alternate calibers like this 300 Blackout to gain improved terminal performance.

6.5 Grendel

While the AR-type rifle in traditional calibers is assumed to be a 400 yards or less cartridge, some enterprising folks have turned a variant into a pretty nifty longer range performer. Bill Alexander and Janne Pohjoispää developed this round as a way to make a standard AR rifle reach out to 800 (and more) yards. It’s designed to fit in a standard AR magazine size, so it gains its extra oomph by using a larger diameter case to launch a long and thin bullet.

Most 6.5 Grendel bullets weigh between 90 and 140 grains, with a common one landing at 123 grains and moving at over 2,500 feet per second. The low-drag projectiles maintain supersonic velocity past 1,000 yards. It also carries over 1,000 foot-pounds of kinetic energy to nearly 450 yards.

Bolt action 6.5 calibers can outperform it, but only because they’re not limited by the same overall length restriction due to the magazine well. You’ll want a longer barrel to take full advantage of this one, and, due to the larger case diameter, you’ll need a new bolt. Even though the cartridges are larger, you can still stuff about 26 of them into a standard size magazine.

6.8 Remington SPC

Here’s another one I really like, although I can’t exactly say why. If “just because” is a good enough reason to like an AR variant, then I’m sticking with that reason.

The 6.8 Remington SPC was developed by some special forces guys, some other folks from the Army Marksmanship Unit, along with some engineers from Remington. The idea was to provide better terminal performance than the 55 and 62 grain 5.56mm NATO ammo being used overseas, especially from short-barreled rifles. Tired of evil d00dz not falling down when shot; they wanted a heavier and larger diameter projectile that could still work in the AR platform and maintain reasonably high-capacity magazines.

The 6.8 Remington SPC cartridge is based off the .30 Remington.

The 6.8 Remington SPC cartridge is based off the .30 Remington.

While the overall length is the same as 5.56mm, so it fits in existing magazine wells, you’ll probably need 6.8 SPC specific magazines for best reliability. That’s what I’ve found anyway. The cartridge case is not based on the 5.56mm, but rather a .30 Remington, so to convert a standard AR, we’re talking about replacing bolt, barrel and, magazine.

While 6.8mm sounds cool and metric, it’s really just a .270 caliber bullet, which actually measures .277 inches in diameter. Because we gun people like to make things confusing, I guess. If you’re a hand loader, .270 bullets are plentiful, even in these times of limited reloading supplies availability.

The “average” 6.8 SPC load contains a 115-grain projectile moving at 2,640 feet per second. Of course, you can load lighter or heaver bullets too. The average load translates to 1,785 foot-pounds of kinetic energy. Our average bullet will stay supersonic out past 850 yards, and bullet drop is still manageable at that distance by many magnified optics, so it has more “reach out and touch” capability than the lighter weight 5.56mm rounds.

5.7x28mm

The AR57 upper receiver actually uses the top-mounted FN PS90 magazine and ejects through the lower receiver magazine well.

The AR57 upper receiver actually uses the top-mounted FN PS90 magazine and ejects through the lower receiver magazine well.

There’s only a little bit of cheating happening with inclusion of this one on the AR variants list. Yes, you can use a standard AR-type lower receiver with the AR57 upper to fire the tiny little FN 5.7x28mm cartridges.

However, it’ll look somewhat non-traditional. The FN magazines for this cartridge, used by the PS90 carbine, store cartridges sideways and rest on top of the upper receiver. The magazine well on the AR lower is actually the ejection port. By the way, this unusual magazine holds 50 rounds.

Ou example cartridge is the SS195LF. This little 28 grain bullet hums along at 2,350 feet per second and delivers 344 foot-pounds of energy.

7.62x39mm

If you’re looking for the ultimate in AR diversity, how about one that accepts it’s traditional “enemy’s” ammo? The 7.62x39mm cartridge has been used in at least 319 trillion AK-47 type rifles worldwide for over 2,000 years. Ammo and magazines are plentiful to say the least. Besides, the 7.62x39mm offers a heavier bullet and more kinetic energy than the 5.56mm.

You can get an upper receiver to handle the comm-block ammo, just be aware that you may have to resort to some tinkering and Dremel tool use to get it humming. The usual reliability problems center around three areas. The AR-type rifle magazine well is straight, which doesn’t always sit well with the tapered cartridge and associated banana clip design of AK magazines. Others have reported problems with feeding related to low gas pressure and have resorted to drilling out the barrel’s gas port. As for the Dremel? The standard AR design uses a split feed ramp which tends to hang up the larger 7.62x39mm cartridges. Industrious shooters have put Dremel to metal to smooth out the feed ramp area into a single ramp.

Or, you can cheat a little bit and pick up a rifle “like” an AR that’s optimized for the cartridge. CMMG’s Mutant and the Rock River Arms LAR-47 both offer lower receivers built specifically for the AK cartridge fan club.

.458 SOCOM

If you favor lobbing cinder blocks down range, you might consider one of the standard AR rifle big boy caliber options, .458 SOCOM.

Like other non-traditional AR calibers, this one was developed after complaints of 5.56mm stopping power. This cartridge swings to the polar opposite end of the slow and heavy versus light and fast scale by using a lengthened .50 AE cartridge case and 300 grain bullet moving at 1,900 feet per second. That generates 2,405 foot-pounds of kinetic energy.

The beauty of this one is near complete compatibility with the standard AR, excepting of course the barrel. You don’t want to try and stuff a .458-inch bullet into a .224 inch hole. Everything else, magazines, bolt, buffer and springs, is the same. [Correction: The bolt is not compatible, that’s a typo!] The big fat cartridges stack singly into a standard AR magazine, so a 30-round 5.56mm magazine ends up holding 10 rounds of .458 SOCOM.

Can you say “infinite bacon?”

.308, 300 AAC Blackout and 5.56mm cartridges.

.308, 300 AAC Blackout and 5.56mm cartridges.

.50 Beowulf

Not to be outdone by tiny little 300-grain, .458-inch bullets, Alexander Arms offers complete rifles and uppers in .50 Beowulf. This one is intended to replicate the famous .45-70 Government round.

If you go with the 400 grain bullet option, that’ll lob down range at 1,800 feet per second and deliver 2,878 foot pounds of energy.

Pistol Cartridges: 9mm, .40 S&W, .45 ACP

Too many to list here, numerous makers offer upper receivers for standard pistol calibers like 9mm, .40 S&W and .45 ACP. If you want the handling and easy sight picture of a carbine, maybe one that matches your handgun caliber is in order? The longer barrel length will give your pistol rounds a little more velocity, and therefore longer effective range.

.22LR

Coincidentally, .22LR has a number of ways to fit into the AR-rifle family. The bore size is pretty darn close to the standard .22LR version anyway. Regardless of the .223 Remington name, the standard AR rifle uses bullets that are .224 inches in diameter. Regular .22LR ammo varies a bit, from .223 to .224 inches in diameter, so you can shoot it through a standard AR barrel with reasonable success. As a result, a bolt and magazine change, combined with a chamber insert, can turn your existing rifle into a .22LR plinker. Of course, dedicated .22LR upper receivers and complete rifles are plentiful too.

The standard AR barrel is close enough in diameter to shoot .22LR ammo through it with a conversion bolt.

The standard AR barrel is close enough in diameter to shoot .22LR ammo through it with a conversion bolt.

.410 Shotgun

In the “why the hell not?” category is the .410 shotshell upper from Safir Arms. The upper connects to standard AR-type lower receiver and lets you whale away with .410 shot shells or slugs. The magazine is proprietary as you might imagine. You can get magazines in 5, 10 and 13 round versions. Why 13? Maybe the better question is, why not?

.50 BMG

In the “Because ‘Murica!” category is the Zel Custom – AR-15/M16 Tactilite upper receiver. This, like most of the others mentioned here, mounts to a standard AR-type lower receiver.

Th .50 BMG cartridge is just a hair longer than the 5.56mm, so there’s not a snowballs chance in hell that one will go through a standard magazine well without melting it first. As a result, this upper receiver is a single shot only, and you load the .50 BMG cartridge through a bolt action in the upper.

A Smith & Wesson M&P 15 VTAC (top) and Colt M4 chambered in .22LR (below)

A Smith & Wesson M&P 15 VTAC (top) and Colt M4 chambered in .22LR (below)

And many, many more…

In a quick and very informal count, I found at least 53 other chamberings for the AR-type rifle, ranging from 5.45x39mm to .30 Carbine. Oh, that M&P 338 I mentioned earlier? ArmaLite makes an AR-10 variant called the AR-30A1, chambered in, you guessed it, .338 Lapua Magnum.

Celebrate diversity? Heck yeah! But ask yourself this question: What do you want this rifle to do? If you’re an experienced shooter, and this won’t be your first AR, then the sky is the limit. If you’re just getting into the black rifle game, consider the benefits of sticking with the tried and true. Nothing wrong with the old 5.56.

More from this Series: https://www.gunsamerica.com/blog/?s=build+an+ar

{ 66 comments… add one }
  • Gary July 7, 2017, 8:41 pm

    I have seen and would like to build an AR in 7.62×25. This can be a blistering round, out of a Tokarev at >1,700 fps, 500 ft lbs at the muzzle. Think what this can do in a 16″ barrel! Accurate round, reloadable, small in size, impressive round to say the least. Anyone know where an upper and a mag well adapter can be purchased?

  • Tshot December 3, 2016, 4:21 pm

    Tom,. I really want multi-application of ammo. I have and love my modle 94s. I really want a 30-30 in and AR platform. Of course I also have other rifles, but the 94s are the only ones I have that “in my mind” are small enough for the AR15 platform. Can or will it be done. 30-30 ammo is cheap compared to 06, 300 mag and 7mm. What say you? BTW thanks for a great article. Tim.

    • TOLOOSE April 4, 2017, 3:38 am

      I agree, I would love and AR with that round……… if someone doesn’t do it I will. I am a machinist after all.

  • Mik piper November 17, 2016, 11:48 am

    Hey, lets try an ar15 muzzle loader. That would get the jaws a flappin’.

  • Rem870 November 10, 2016, 3:14 pm

    That is the best article about AR-15 calibers. It has very useful information about all calibers available. I didn’t know that .50 BMG upper receiver exists! Thanks for putting this all together.

  • BHG September 21, 2016, 2:09 am

    I specifically came here to read the specifics on the 9mm, .40, & .45…. Got all caught up in reading alllll the other calibers just to find no 9, 40, & 45…. -_-

    Very nice article on everything else tho lol

  • Bob September 18, 2016, 10:53 am

    I recently put a Daytona Tactical 16″ 7.62×39 upper on a PSA lower and it runs everything I’ve tried smoothly with no light strikes or any other problems (so far, only about 100 rds. through it checking out different ammo). Open sights on a rest I’ve only played with it at 25 yds. but with steel cased Red Army 123 gr. balls it grouped .75 in. with 3 touching so I think it’s going to do what I wanted it for. I think I’ve got around $750 in it with a PSA carry handle, Troy front sight and 3 ACS mags. I was a little skeptical of the DT upper, I’d heard very mixed comments about them but my only complaint would be the build time. However, when I emailed them and asked when it would be shipped I got answers from both of their principals within minutes. The upper seems to be solid.

  • Chris September 13, 2016, 1:46 am

    Great article just one inaccuracy.

    “The thinking was that the average soldier could HUMP a lot more cartridges if they were smaller and lighter.”

    It’s the uglier and heavier ones that the average soldiers can hump easily.

  • Nicholas Johnson August 29, 2016, 2:14 pm

    “The 7.62x39mm cartridge has been used in at least 319 trillion AK-47 type rifles worldwide for over 2,000 years” 😛 This actually made me LOL! Nice to see some humor once in a while.

  • chris August 9, 2016, 3:54 pm

    im thinking about building an ar in 7mm mag does anyone know if it is possable and have any links to get the hardware thanks so much i looked everywere online and cant seem to find anything

    • JUNO October 9, 2016, 9:01 pm

      Saw them on YouTube on “offtheranch” and “demolitionranch”

      I would call F1 and ask I am looking for something similar.

  • Ralph April 6, 2016, 9:10 am

    I have built several ARs in various calibers. What I have right now is 2 in 223, one in 7.62 X 39 and one in 6.5 Grendel. The 6.5 is hands down my favorite. Never a bobble, ftf or jamb of any type. Hornady factory 123gr. ammo is all I shoot and is readily available and deadly accurate. I did put one together in 308 but found it way too heavy for my tastes. Instead I put a tricked out HK G3 together which I really like.

  • John February 18, 2016, 10:03 am

    I have two are ARs both the 556\223 just good basic units and I love to practice with them I’d like to thank all! For the info here. Some of this is above my head. So I need all the info I can get

  • Danny smith February 15, 2016, 10:25 pm

    Will 7.62x39mm rounds fit in an AR15 5.56mm magazine?

    • Wes March 26, 2016, 2:53 pm

      No

    • Brian Doughty April 1, 2016, 3:39 pm

      Yes , they actually will if you modify the ft lip of the magazine lower so the case doesnt snag on the ft lip, but its better to use a 7.62×39 dedicated magazine.

  • Thor August 13, 2015, 1:47 pm

    I know the original article concentrated on the wimpy AR 15 (not even legal in many states for deer much less wapiti or bears) so I started with the AR 10 (never heard of the ought-six possibility in semi-auto) and then built a .338 Federal upper. Same bolt as .308 as it is a necked up .308 case. Where is the .338 Federal? It’s like a mag version but not a big propellant consumer and those big slugs (many different weights available) retain velocity & energy at distance.

    • David Bennett June 1, 2016, 5:05 pm

      There is a company called Noreen that offers a .30-06 AR that is purpose built. I have never shot one, so I don’t know how well they perform. It seems like a .308 winchester can do nearly all the things a .30-06 can, so when I’m able to get a mid range AR, I’ll be going for the .308 option…

  • Rick July 20, 2015, 9:42 am

    Re:5.7 x 28

    If the unusual profile bothers you, why not take a beater 20 round 223 mag, cut or reform the feed lips and leave the floor plate off.

    You could even expand it into a brass catcher!

  • Deef David July 14, 2015, 3:44 am

    From years of reloading and shooting enjoyment shoot what ever you want, experiment with what ever you can come up with but remember when the s**t hits the fan military ammo of your resident country’s armed forces will be easier to get your hands on. Ammo is heavy so if you are not sitting around guarding your stash you will have to carry it which for most of us will happen!

  • JediPirateNinjaMike July 6, 2015, 7:31 pm

    The AR-30A1 is not an AR-10 variant.

  • BIGDADDY June 30, 2015, 11:30 pm

    I LIKE THE 204 FOR COYOYES OVER THE REST THAT YOU MENTION. I RELOAD 8 DIFF. CALIBERS IN AR’S AND IT IS MY FAVORITE.

  • Joseph L. June 29, 2015, 6:06 pm

    Hey you guys, I find the best solution is to have 2 ARs, a 5.56 and a 7.62. The reason is simple. These two rounds will be in service long after we all are dead and gone. When the ZOMBIES come calling I hope I am not crawling around looking for some exotic round when all I had to do was go to the nearest Dept. Store and grab something you know will be in stock. At this time I have over 10,000 rounds of both and I also have rifles(at least two) that can handle .223/5.56 Nato and the .308/7.62 Nato. Just point me in the right direction!!!

    • Donald July 13, 2015, 2:07 pm

      The AR15 in 5.56 and 7.62 is the way I went. SOTA Arms upper and New Frontier Armory lower. Have fired about 1000 rounds in each without a problem. I also have a Mini 14 and Mini 30 which also uses the 2 calibers. These 2 calibers are the best way to go because ammo will always be available.

  • Joseph L. June 29, 2015, 6:05 pm

    Hey you guys, I find the best solution is to have 2 ARs, a 5.56 and a 7.62. The reason is simple. These two rounds will be in service long after we all are dead and gone. When the ZOMBIES come calling I hope I am not crawling around looking for some exotic round when all I had to do was go to the nearest Dept. Store and grab something you know will be in stock. At this time I have over 10,000 rounds of both and I also have rifles(at least two) that can handle .223/5.56 Nato and the .308/7.62 Nato. Just point me in the right direction!!!

  • Ratech June 29, 2015, 4:57 pm

    “The 7.62x39mm cartridge has been used in at least 319 trillion AK-47 type rifles worldwide for over 2,000 years. Ammo and magazines are plentiful to say the least”.

    Huh? What is this nonsense? there have been approximately 100 million ( +/- a million or so) AK-47’s and variants made thus far. The gun was manufactured in the mid to late 1940 so that make it approximately 70 years old in 2015.
    https://en.wikipedia.org/wiki/AK-47

    So where are the facts to back up the 319 Trillion and 2000 years?

    Maybe he means 319 trillion rounds of ammo has been fired through the AK-47? The sentence clearly states the cartridge has been used in 319 trillion AK-47’s so maybe he means Million and not Trillion?

    The 2000 year statement still baffles me. I guess Ancient Rome had AK-47’s and we didn’t even know!

    Hey Tom have someone proof read your work for errors before publishing please.

    • Hank Salley June 29, 2015, 9:42 pm

      How is it everyone is taking these numbers seriously? It is quite obvious that this was a tongue-in-cheek reference to the large numbers (maybe a “gazillion” would have worked better) of AKs in use since it’s inception a “bazillion” years ago. Wake up guys!

    • J.Cline June 29, 2015, 10:19 pm

      I believe he was exaggerating/joking brother calm down lol

    • Mark June 29, 2015, 10:31 pm

      Seriously?!? Does nobody understand that he was being exaggerative, sarcastic, and humorous? All of you so ultra serious and suffering so dearly from rectal-cranial insertion syndrome that you can’t understand a joke?
      Relax, boys.

    • Lifeisdeath June 30, 2015, 12:29 am

      its was a freaking joke meant to highlight the fact that everyone uses it and has been around forever. also an exaggeration. wow.

    • Tom McHale July 1, 2015, 9:01 pm

      You know what? I double fact checked, and you’re right. There are are only 3.1785592 Trillion AK rifles in existence. The other 2 were actually lost on Noah’s Ark. Most historians suspect that the pair of Spider Monkey’s on board heisted them.

      You also got me on the 2,000 year old fact. I checked, and there were actually invented by the Roman Emperor Elagabalus in AD 221 for use in high-volume porcupine hunting. So that makes them only 1,794 years old.

      I apologize for the mistakes!

      • DRAINO July 2, 2015, 9:47 am

        LOL!!! WOW! Like watching reality(not so much) TV. You don’t have to worry about making things dramatic…..people will do that automatically…LOL!

    • Chris September 1, 2016, 10:56 am

      It was an conscience exaggeration….he was KIDDING…

    • Jerry Ward November 4, 2016, 1:44 pm

      Its a joke

  • George Reed June 29, 2015, 12:11 pm

    How about the .30 Remington AR? A 150-155 gr bullet at 2600 fps from a 22 in barrel from an AR platform, My R-15 shoots 1 in groups at 100 yds with factory ammo. It does require a special bolt, though.

  • stravo lukos June 29, 2015, 8:43 am

    Nice articles, a few typos, nothing worth an article 15; however, I do have a bur under my saddle. Doesn’t it bother anyone else to see a hand-assist on a rifle, esp. one on which your life might depend? It takes me back to the days of the Mattie Mattel-16 & grunts having to use their rifles as high-tech clubs. If memory serves me properly, Stoner used the tried & true twin lug bolt on his original design w/ no problems. He also used a slow twist for extra instability. There are two good suggestions for another few articles, GA.

    • s. herst June 29, 2015, 7:06 pm

      The original AR-15 used a 1-in-14″ twist for the instability you describe, but in very cold winter field testing, it was discovered that the twist was TOO slow, causing bullets to keyhole even at short ranges and lose accuracy when the weather was extremely cold, though not at normal temps.. The solution at that time, was to use a 1-in-12″ twist for the then-current 55 gr. bullets, which were heavier and longer than the then-normal 45 or 50 grain bullets of the .222 Remington and .222.Rem. Magnum, for which the commercial 1-in-14″ twist that had been standard for those calibers. Note: longer bullets require faster twist to stabilize. So the military decided they needed a twist rate that would work at any temp likely to be found on earth, even if it gave up a slight degree of tumbling or fragmenting..

      When the military changed to 62 or 63 grain bullets in the M855 round (NATO SS-109), they were even longer than the 55’s, so a faster twist was needed, so by default they went to the same 1-in-7″ twist already used in the M-249 LMG. That was actually TOO fast a twist rate, since the M-249 was optimized for the M-856 tracer round which was REALLY long and needed the 1-in-7. The too fast twist left the M-16 and M4 with less tumble or frangibility, thus causing some of the recent complaints about “lack of stopping power”. Which brings us full circle to the rationale for developing 6.8 SPC and the .300 Blackout and 6.5 Grendel in the search for heavier bullets and more “stopping power”. Some of the extra-heavy, extra-long 75-87 grain target bullets (may not feed from mag) in .223/5.56×51 work best with the 1-in-7″ twist, since their bullet lengths more closely approximate the length of the M856 tracer round. In some cases, the best performance with the 62-63 grain M855 (or SS109 and similar rounds) will accompany a 1-in-9″ or 1-in-8″ twist rather than the now common factory 1-in-7. The added advantage of a slightly slower twist is longer barrel life (Colt notified the DOD that their required 1-in-7″ twist was going to cause fast barrel wear, but the bean-counters liked the idea of an M-16 that could use the M856 tracers in a pinch….even though it was almost never done, though tracers could be used if they had some of the older M196 tracers for special missions with the 1-in-9 barrels. But that would be adding to the ammo to be stocked and transported. The bean-counters prefer Commonality rather than effectiveness.

    • Tom April 11, 2016, 12:50 pm

      This is all well and good, and fun to “What if” all the possibilities to death… Here is one cold hard fact. I’m a 68 year old disabled Vietnam Veteran. Everyday I am reminded of at least one, of the times I did something stupid almost 50 years ago. One anti-tank mine, and 2 bullet wounds were not enough, I had to volunteer for a 2nd tour on a Huey Gun Ship as a Door Gunner. Nice, clean, lots better views than the cramped enemy tunnels I’d been in as a Tunnel Rat.
      Finally after 158 combat missions in the air, and more action in 3 days in the air, than I’d experienced in 11 months as a Cavalry Recon Scout…and two helicopter crashes, I finally went home.

      What are my worst nightmares ? That POS M-16 going “click”, the frantic jerk on the charging handle that ripped off the back of the cartridge. What now ? No battlefield solution here. Once in a 9 hour firefight in our Assault Vehicles I had to drive as the driver was hit. I jammed all 4 of the crews M-16’s. All spotless not one even making it through one clip without jamming.

      So, before you all get all high tech, remember the first rule of Combat “KEEP IT SIMPLE, STUPID”. The KISS principle.
      Things are supposedly a lot better now, 50 years later. Of course the Military still runs their weapons division on Bribes & Kickbacks. They got rid of the nearly perfect 1911, and gave us the POS 9mm Beretta which took at least 3 rounds to stop an enemy. More than enough time for him to spray you and your buddies on full auto as he hit the ground.

      What are they doing ? Their deciding on a new 1911 “looking” replacement Military Pistol. The Marines in their usual common sense approach…just ordered 6000 1911’s. The SEALS use 1911’s, the SF’s guys like their 1911’s, too. Oh, and the Army just spend $44 million dollars on a new Sniper Rifle by H&K that looks like a M4 only a little longer and in 308.

      They’d become worried that the soldiers Sniper Rifles will give them away while he’s walking around with the Grunts, That our Snipers would draw unnecessary fire. So, they’ve spent $12,000 each for new M4 look a likes. No one told them apparently about the A-10.

      BTW, not only do we already have M-4 look alikes in 308. But we have a US Company that makes a full size M-16 type Sniper Rifle in 300 Winchester Magnum guaranteed 4″ MOA at 1000 yards. Not even considered.

      Play all you want, there may come a time when you only have two choices in available ammo. 5.56 & 308. You have an AR, Mossberg makes a Bolt Action Rifle with 3/8th MOA in 308 with a fluted barrel, spiral grooved bolt, nice trigger, composite stock 6.75lbs …and it can shoot with my old $3000 Custom Rig.

      Want to stay alive ? KISS….Keep it simple. Spend the money you were going to spend to have something “cool” for more ammo. Use that additional ammo for the “practice” that might save the lives of you and your family.

      I’m an old man now, I survived when so many others did not. My Gun Ship had 4 different door gunners in 9 months. I fought throughout the TET Offensive of 1968. My M60 was stripped down and souped up to over 1200 rounds per minute. On an “average day” I fired easily 10,000 rounds. My records long day was 25,000 rounds. We painted our confirmed kills in red skeletons on the side of the Gun Ship…it was like honey to flies.

      They’d step out from behind cover young ego’s. We were the kill ship, the one with the recognizable round ball on the nose. The 40mm nose canon Gun Ship. Everybody wanted a piece of us. The 40mm rounds only had a range of 300 yards so attacks were low level, like 20′ off the ground low.

      The KISS principle in practice…nothing special, weapons clean, tricked but only possible with wind speed and rotar wash. Keep it simple, and never Rush to Die. Let the enemy rush to you to die…

      The new 5.56 ammo is totally different that crap we used in 1967. The secret is not some fancy new cartridge, take it from someone who’s been there done that. I can’t even count the number of wounds I survived, I can see the scars, I can still feel the pain from some of them…but out of the original 8 of us in 1st platoon, I was one of the only 3 who survived after TET 1968. But we took out thousands of Viet Cong… sometimes you just can’t beat the odds, everytime.

      Here is my suggestion to ya’ll… pick your Rifle, pick you barrel length, Contact La Rue Tactical out of Austin Tx. They had a sale months ago and I bought a few trigger groups for my AR’s ranging from 2.5lbs to 4.5lbs. Sniper & Tactical. Triggers are the heart to accuracy.

      Next in the SIMPLE PROCESS. Match your ammo to your intended mission. Home defense, light fragile bullets that won’t over penetrate. slow twist barrels, bullet weights around 40 grs. around 4000fps. shouldn’t go through walls, but very messy on whomever takes one in the chest, or arm. Either way the bullet explodes.

      Longer shot. 1:7 twist and 77 gr. bullets makes the 5.56 an accurate decent long range Sniper Rifle. The trick is 77gr is ideal, but if you run out of ammo, you can reach down and pick up a 55 grain and it will still shoot and kill someone. SIMPLE.

      KISS…you have optimal and you have acceptable…both are lethal. Running out of ammo is neither.

      Save your money, instead of buying new toys…buy a new upper, long barrel with 1:7 twist for 40 grain speedsters that explode on impact. Talk about a moral beaker ? Hit a 20ish young man in the groin with a 40 gr, 5.56 JHP. All his buddies will hear his screams and see the grapefruit size hole where his genitals use to be… suddenly you have instilled “fear” in the place of where invincibility just moved out of….

      Choose your barrel, twist, length and match them with the right bullet. Then practice until your lethal. That’s how you stay alive…and that’s how they “don’t”.

      Gentlemen, that is KISS. Purpose, Barrel, Length, Twist, Bullet & Weight. These are the the pieces of your trade. Assemble them in correct order, practice and you become the most feared individual to the enemy. When they put a “bounty” as they did Blaine Shepherd and I ….your there. Of course ours was only $10K each, for our heads & d tags but the average Vietnamese farmer only made $200 a year. So everyone shot at the Gun Ship with the round ball in its nose.

      Earn the bounty … KISS will keep you alive long enough to impress your kids.

      • Charles Scruggs (Greyhound 3a) August 10, 2016, 2:02 pm

        Excellent article. Do I know you sir. Your brother always. Above The Best

      • Levi September 1, 2016, 10:44 pm

        Are you still around Sir?

      • R. G. Vest November 21, 2016, 9:29 pm

        Thanks

  • Chester Carstairs June 29, 2015, 7:46 am

    Good write up. However I don’t think the 7.62×39 has been around 2,000 years yet. lol

  • Joe June 29, 2015, 7:30 am

    After having poured over the various AR variant offerings for a decade before surrendering my beloved Mini 14 to my nephew and purchasing what I thought to be my last bucket list rifle, the main factor in my choice was the cost of store bought ammo and reloading the various offerings with my own equipment. Also knock down power came to mind as well.
    My first thought was the Colt AR 10 offering with the mag well insert and a separate upper for the 5.56 NATO round BUT the twenty two hundred dollar layout and the demise of COLT quality shot this one down in flames.
    So I ended up with a bushmaster carbine in 5.56 and a PA 10 in .308 NATO ( I really wanted one in 30 06 but the cost again got to me).
    The 5.56 NATO round is plentiful and affordable not to mention easy on the old shoulder, and great for varmints and target shooting,
    The .308 although not as affordable as the surplus 30 06 ammo still has the knock down power as a heavy hitting round needs for those big hogs running around the deep south, and growing numbers of black bear too (although I haven’t had either of them knock on my door yet).
    Sorry to say the safe is full and I’m tapped out.

    • Robert June 29, 2015, 8:51 am

      Joe, LOVE the idea of an AR chambered in 30-06. Is someone manufacturing the parts for this? What were you looking at for cost?

      • Ken Myers June 29, 2015, 9:43 am

        Robert, I have multiple rifles in .30-06 from WWI to present and I have been looking at this one.
        http://onlylongrange.com/bn36-carbine-assassin/
        Good luck with your choice… 🙂

      • Joe June 29, 2015, 10:18 am

        Sorry I don’t recall the brand but it was a complete rifle, not a kit and it was upwards of two grand. It looks like somebody gave a link for you to view. GOOD SHOOTING.

      • Gary September 3, 2015, 9:47 am

        Yes, I believe Noreen does along much bigger!!!

    • Tom McHale June 29, 2015, 11:47 am

      Random thought, but I could have sworn I saw one in 300 Win Mag at an NRA Annual Meeting a couple years back. Now that would be a fun rifle 🙂

      • Joe June 29, 2015, 12:39 pm

        I’ve seen uppers in .338 Lapua and .50 BMG only in bolt action format so the 300 MAG which is just a 30 06 on steroids is doable in a priority format in semi auto.

      • Jeremy June 29, 2015, 1:14 pm

        Nemoarms has a number of 300 win mag models as well as a 338 win mag model

        https://nemoarms.com/rifles/

      • JZ January 26, 2016, 8:08 am

        POF just announced at shot show 16′ they are producing a piston AR in 300 win mag.

    • Tom April 11, 2016, 12:53 pm

      Are you aware there is a US company that makes an AR in 300 Winchester Magnum ? Its a beauty and they guarantee 4″ moa at 1000 yards.
      Kinda skips over the old 30-06 and goes directly to the big league without the big ammo price tag.

  • Forrest Ebet June 29, 2015, 7:26 am

    Good article but you may want to include the 6mm Ar based off the 6.5 Grendel case. Yes it’s a hand loading option only but with the vast array of .243 bullets and it’s inherent accuracy it comes very close to .308 performance.

  • Bartt June 29, 2015, 7:23 am

    You seriously missed the boat on the .458 socom. It requires a specialized bolt and extractor and substantial modification to the ejection port.
    -Bartt

  • SmokyBaer June 29, 2015, 5:43 am

    Just to note…
    My 458 SOCOM AR build required a special bolt.
    You may want to review your comment that it uses the standard 223 jammer.

    Keep shootin straight!!
    John

    • Tom McHale June 29, 2015, 9:35 am

      Thanks for the catch, I know that and made a typo on including the bolt on the list of things that are cross compatible. That .458 cartridge rim is “just a hair bigger” than the .223, isn’t it 🙂

    • TALON June 29, 2015, 11:50 am

      HELLO
      I DID NOT KNOW THE AK47 HAS BEEN AROUND FOR 2000 YEARS

      • Terry June 29, 2015, 5:18 pm

        Unlike the AR (Assault Rock) that has been. 🙂

    • TALON June 29, 2015, 11:52 am

      I HEARD THAT THE COAST GUARD WAS HAVEING A PROBLEM WITH THE 458 SOCOM WITH CRACKS IN THE LOWER & UPPER
      I THINK IT SHOULD HAVE BEEN BUILT ON THE AR-10 PLATFORM

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