One Lower – AR Mags, AK Mags, 9mm Mags – Windham RMCS-4 – Full Review.

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During the late 1980s and early 90s, Bushmaster was the only real alternative to the Colt if you wanted an AR-15 rifle. They were a real company that made stuff, not just a business ordering parts with their name on them. Bushmaster was based in Windham Maine, and for many years they created innovative products for civilian, law enforcement and militaries across the globe. In 2006, Bushmaster was sold to Cerberus Capital Management, which was named after the mythological three-headed dog that guarded the gates of Hades. As part of this deal, they were later packed up from Maine and moved, lock stock and barrel, to New York.

The RMCS-4 comes packed in a custom foam cut Plano case that holds all the parts needed for the four chamberings.

About the time the company was moved to New York, the previous owner Mr. Richard E. Dyke, had his non-compete agreement expire. Mr. Dyke also owned the land and facilities that were the longtime home of Bushmaster. Something else that Cerberus had left in Mane was the skilled group of people who knew how to build AR-15s.

A few people were engaged and the next thing you knew, the band was getting back together! In three short years, they were shipping rifles again, this time with the credo “The Quality Goes In, Before The Rifle Goes Out” under the brand Windham Weaponry.

I, for one, am glad to see a quality manufacturer back on the market!

The complete RMCS-4 kit provides you with a reliable four-caliber AR-pattern system that is ready to go right out of the box.

More History

At the 2016 SHOT Show, Windham Weaponry announced that they would be showing a groundbreaking new rifle that would allow the user to convert between calibers by changing out the barrel, mag well, and bolt carrier group—all while utilizing the same lower and upper receivers. They were going to have three versions available, with up to four caliber options in one AR platform rifle. These caliber options would be .223, .300 BLK, 7.62×39 and 9×19. Windham would swap in a CMMG .22 conversion kit in place of the .300 BLK if desired. Today, we are considering the RMCS-4, with all four (.223, .300 BLK, 7.62x39mm and 9mm) chamberings included in the kit.

The heart of the carbine’s multi-caliber system is the modular, swappable magwell system.


  • Chambering: .223, 300 BLK, 7.62x39mm and 9mm
  • Barrel: 16.12 inches
  • OA Length: 32.5 to 36.25 inches
  • Weight: 7 pounds
  • Stock: Collapsible
  • Sights: None
  • Action: Direct gas impingement
  • Finish: Matte black
  • Capacity: Variable
  • MSRP: $2,971 (as tested)


When the discrete cardboard box arrived, I popped it open and out slid a lockable Plano Molding 36” All-Weather Storage Case, with a custom-cut Mil Spec foam interior. Within said storage case was held a bounty of hardware. I glanced around and saw a complete M4 rifle without sights or optic, three magazines, three extra barrels, three total magazine wells, three total bolts and a sling. I was impressed with the package; it looked like a functional layout for field use.

The lower receiver of the RMCS is split into two parts—the forward modular mag well and the rear half with which it interfaces.

After looking through the manual that was included, I figured out that step one would be to locate the multi-tool that had come with everything else. The multi-tool served as a chamber indicator, punch to push the pins out, and a pin to depress the detent on the trigger guard. This tool would be more than handy for smooth caliber changes.

The next big standout is the barrel change method. The key to barrel interchangeability is on the underside of the MCS Upper Receiver. Once the Barrel Retaining Block is unlatched and moved forward, the Barrel Retaining Arms rotate out to release the barrel and gas tube. The new barrel is inserted and the process is reversed to secure it in place. All the barrels are machined from 4150 Chrome Moly Vanadium Steel, chrome-lined (except the 9mm Barrel, which is Melonite finished) and clearly stamped with caliber for clear selection.

The included multi-tool is used to push the pins holding the upper and lower receiver together. Once the upper and lower receiver are separated, changing the bolt (if necessary; .223 and 300 BLK share the same bolt) is standard AR-15 fare. All the bolts are machined from Carpenter 158 steel with staked gas keys.

The .223/300 bolt carrier has rings behind the lugs to differentiate it from the similar-looking 7.62x39mm bolt carrier group.

The RMCS-4 comes with three bolt carrier groups: One for the 7.62×39, one for both .223 and 300 BLK, and one for 9mm.

Finally, the coolest part of the concept is the modular receiver and magazine well, which are CNC machined from forged 7075 T6 aircraft aluminum. The process of changing the magazine well begins with extending the pin on the orange multi-tool, then depressing the detent holding the trigger guard. The trigger guard is swiveled down; this mechanism is designed to permit a gloved hand to fire the gun. Finally, the magazine release is depressed, and the magazine well slides up and off of the lower receiver.

This process works the same for every caliber change. I found that once I was comfortable with the process, it took some two minutes to go from shooting one caliber to shooting the next.

The low-profile AK mag well is shown here. Note that the standard AR-style push button release is used rather than an AK-style paddle/lever.

There are a few details I would caution you about to avoid a disaster. Note that the conversion from .223 to 300 Blackout requires only a barrel change. The bolt and magazine remain the same, which is convenient, but also creates a potential risk. One could end up converting the gun while leaving the original ammunition loaded—this could be a very dangerous situation. I have started painting 300 Blackout magazines or marking them with tape, to avoid grabbing a .223 by mistake. Also, note that there are three bolts used between all four calibers, and as I noted above the .223 and 300 Blackout share the same bolt. The 9mm bolt is easy to distinguish, but the 7.62×39 bolt looks remarkably like the .223 bolt. The .223/300 Blackout bolt is marked with lines below the lugs, cut around the circumference of the bolt, allowing you to confirm that you have the correct choice. I would have preferred a different coating that would have resulted in a different color, such as nickel boron. This could have added another layer of safety to the kit.


The first rubric of specification for the RMCS-4 is broken down based on what is common to all iterations of the kit. The next table of specifications is specific to each caliber option.

While prepping for my trip to the range, I tried to consider the person who is looking for one gun that shoots four calibers. This could be for any number of reasons that I will discuss later. The real rub come testing time would be point of impact across calibers. Could this be done accurately, with the optic remaining the same across the barrel, bolt, magazine and magazine well changes? To test this, I selected the Trijicon MRO with the see-through riser mount. This red dot with the riser would allow for back up iron sights to be installed and used without change. I felt sure that I was giving up the detail and accuracy that a magnified optic would afford me. However, I felt it was worth the potential sacrifice given the range of calibers and configurations that needed to be tested to truly cover the platform. I was happy to discover that the rifle still fit into the supplied case after the Trijicon MRO was installed.Rationale and range prep

I selected three different brands of ammunition to allow for reasonable variances that would be expected in a normal range session. This platform would not be practical if the trajectory of projectiles had to be matched.
After arriving at the range, I had to wait a bit to get an open bay to shoot in. I had 100 yards to work with, which would be no challenge for 3 of the 4 cartridges I was going to fire. To accommodate the one pistol caliber, I settled on 50 yards. I knew that the 9mm round will easily travel 100 yards, but the drop would be around 7 inches, throwing off the point of aim when comparing to the other rounds.

Range Time

First up was the .223; after about 10 rounds to zero in I fired a 3-round group for accuracy. I changed to the 300 Blackout next, as this only required a barrel change. I again fired a three-round group. I followed this protocol until I had data on all four iterations. There was only one notable thing that revealed itself during the range session: the 7.62X39 option had no provision to lock the bolt back on an empty chamber.

The resulting data listed above allowed me to draw a few conclusions. First, you could use one optic for all four rounds without significant changes, albeit a red dot. Second, the most accurate configuration was hands-down the .223 round. The point of impact changes between rounds were noticeable, however, I could run the same optic without adjustments on a man-sized target.

The reliability of the gun was 100 percent, which was noteworthy considering that some guns seem to be challenged just to run one caliber reliably, much less four. The transition between calibers was easily done by utilizing the supplied case, and did not require any special accommodations. The supplied magazines were well-chosen and did not present any performance issues.

The one fault I would offer is that the trigger was a bit stiff, and this rifle particularly could have stood to have a smoother trigger. The trigger and my use of a red dot over a magnified optic both detracted from my shooting, and I am quite sure that I did not close in on the maximum mechanical accuracy of this rifle.

My Bottom Line?

The platform works well, both as a rifle and as a system. The RMCS-4 is a truly unique concept, but are these factors enough to justify owning it? I think the answer may be yes, especially when considering some other possible scenarios. If you are going to shoot a match in several divisions, requiring several different calibers, this may be the gun for you. If you live in a state where you can only purchase a limited number of guns within a certain timeframe, this could afford you an option. If space is at a premium, and you have the option of adjusting your optic between caliber changes, this is certainly for you!

The RMCS-5 comes with an AK mag from Magpul (left), an AR mag for the .223 and 300 BLK (center), and a 9mm “stick” mag (right).

The heart of the switch barrel system is the dual barrel retaining arm system located on the bottom of the handguard.

Plus, if you want to be able to tell your wife that you only own one AR-15, while also having four options, this gun will allow you to do that! The price offers some economy of scale; if you compare the quality of components and consider that you’ll only need one optic rather than four high-quality optics, you start to see some real value. Or, you could just get it because it is cool!

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  • loupgarous February 14, 2018, 1:26 pm

    The generic issue with multi-caliber carbines is that while nifty, they often cost more than several carbines, each one chambered for a different round. For $2,971 one can buy a 9mm carbine, a carbine in 5.56mm. and a rifle in 7.62mm (if you look around) each of which has AR furniture – its own barrel, upper, lower and stock. so that changing optics and accessories between them isn’t rocket science.
    Now, as far as the reasons why you’d want a system that made you share a single stock, upper and lower receiver, space is the only plausible one – if you travel a lot and don’t want to lug four gun cases (and for some reason known only to God, need all four chamberings where you’re going), yes, this is the gun for you.
    But that stuff about “if you want to be able to tell your wife that you only own one AR-15, while also having four options, this gun will allow you to do that!”?
    MY wife would look at the $3000 price tag (well over that, once you pay your FFL dealer for his trouble and shipping/handling) and ask “where’s mine?” And she’d be right – we’d be able to buy three rifles (a 9mm carbine, rifles in 5.56mm and 7.62mm NATO) for the price of this system, and each have at least one working weapon when the SHTF. Because zombies won’t kill themselves.

    • Thunder June 15, 2019, 11:52 am

      Oh , so true ! At 3k with today’s prices on the down turn it’s just too costly to own and justify the expense

  • loupgarous February 14, 2018, 1:12 pm

    Nitpick: “When the discrete cardboard box arrived…” You probably meant “discreet” (a box that didn’t scream “I’ve got a GUN inside”), not “discrete” (“individually separate and distinct.”, not one of many cardboard boxes you had to open up to find the one with a rifle inside).
    It’s a common error, but one that you easily avoid. (Like confusing “too”, “to” and “two”.)

  • AZArchangel55 January 4, 2017, 6:07 pm

    WOWZA!! Tuff CROWD!! I bought a new Windham shortly after they were available and choose to shoot it over my Colt for range time, hunting, and just shooting. I really like it, great fit and finish and have never had any issues with it. That being said, I like the concept of 4 in one, but it seems like a bit of a novelty. The only way I would be able to justify it, is the consistency of shooting the same platform, optics savings and being able to shoot more without regard of ammo costs or shortages. Plus it is just plain COOL!! If money were no object, I’d buy one!! But as I said in the beginning, I’m a Windham fan!!

  • Douglas Riding January 3, 2017, 11:46 am

    I say: Why buy one rifle when 4 ( or 5 ) rifles will do ???
    Where is the fun, or utility for that matter, in all of this ???

  • Thunder January 2, 2017, 9:43 pm

    Oddly , seems like a knockoff of the MGI Hydra . Nothing new here .

    • Thunder June 15, 2019, 11:57 am

      Ive owned the MGI since they 1st came out . I’ve used the MGI as a Back Pack carbine for barrel length way back when PSD Contractors could take over their own Firearms & bring them back & used in the Mountains . This Windham is just too costly of a real life Carbine .

  • Chris N January 2, 2017, 2:49 pm

    I agree with a couple of the other comments. This is nothing new; it is a complete clone of the MGI system and if they are not already affiliated with MGI they will most likely be sued for copyright infringement in the near future.

  • Franke January 2, 2017, 10:41 am

    Uh, has anyone thought about having to sight in after each barrel change? Maybe even change optics for the task? Have they considered the possible issues of wrong bolt in the weapon when needed, or wrong mags when you get to the range? Too expensive, and too much chance of missed steps in conversion or gear swap. Sorry, but I will keep my separate weapons for each caliber; and know what I have when I pick it up and which mags to grab. For that kind of money I will keep my nicely outfitted AR and my Beretta CX-4 carbine and don’t even want a 7.62 x 39. (for that I prefer a .308 anyway)

  • flintman50 January 2, 2017, 9:16 am

    What a waste of good “gun” money…..

  • Smitty January 2, 2017, 8:46 am

    This is nothing new or innovative. This is MGI’s system that has been around for a very long time. For what ever reason it never caught on. I remember when MGI was making it there were a lot more caliber choices.

    • Gibson January 2, 2017, 3:14 pm

      I totally agree with you. This is nothing more than a copy of MGI’s Hydra. MGI is still around and making them.

    • billy blyth January 2, 2017, 4:01 pm

      Problem is, nobody has ever seen or heard of a MGI. Where as I know guys that own Windhams. If I was only going own one AR style rifle, this one would be it.

  • Pete January 2, 2017, 7:05 am

    The author admits he could have shot more carefully. Otherwise the shooting test was disappointing. And $3,000.00 is a bit pricey considering you could by ARs in the four calibers, one of each, for less. And for those of us who can’t afford an Aimpoint, not having to change optics among the rifle calibers is break-even at best.

  • Vince W January 2, 2017, 6:51 am

    Can anyone say “MGI”?

  • Kevin January 2, 2017, 6:41 am

    Throughout all of last year, this writer always uses the manufacturer’s buzz words, won’t objectively review anything, passes his hour or so at the range as “extensive” and seems to be schilling for each gun maker as innovative. I know these spots are bought and paid for, but put some effort into making us think otherwise.

    • Thunder January 2, 2017, 9:53 pm

      Reminds me of my 6th grade shop teacher . MGI makes the original , so nice that I own 2 . 1 reg.SBR / work carbine and 1 for sport . 3k is too much compared to the MGI ., This Author is uninspiring at best

  • LouisianaMan January 2, 2017, 6:34 am

    Wow, I’m impressed! And that comment stands for the weapon system, written review, and range report.

    Sure, you didn’t perform a full-scale torture test to see how the system holds up under the stress of thousands of rounds, but then again, you’re not the US Army! Time will tell, and others will determine that. Your test explains the concept and shows that it WORKS, and it’s up to the individual consumer to determine whether this 4-caliber arrangement suits his fancy. There are lots of reasons to find this multi-caliber platform attractive, whether based on utility, simple attraction, or a combination thereof. We’ve seen ammo availability hit peaks and valleys during recent years, and today’s plentiful availability does not erase that fact from consideration. Seems like a shooter could get a LOT of familiarity with this rifle using inexpensive 9mm FMJ ammo, then switch to harder-hitting calibers whenever desired. In any type of potential emergency situation, it’s intuitively beneficial to be able to use four commonly available calibers and switch if & when necessary. Somebody might wish to use the 9mm for house defense, and swap calibers for field or emergency uses which make the other calibers more advantageous. I’ll leave those endless possibilities to the reader’s imagination. Think: varmint control, hunting, civil unrest, etc.

    Some interchangeability of POI is a huge bonus in my eyes, not a bottom-line necessity. Rapidly re-zeroing between calibers could be accomplished relatively easily under any imaginable field conditions.

    I don’t shoot competition, and will leave it to those who do to determine the relative value of this rifle in matches. Practical working functionality and accuracy are my primary considerations, and this system has it. Of course the 7.62x39mm results are puzzling and don’t seem to match the performance of everything else, but that may be remedied by a quick factory exchange of an out-of-spec barrel, use of different ammo, or whatever. Worrisome? Yes, of course. Fixable? Highly likely.

    Windham’s mechanical ingenuity is worthy of the highest praise! Many of us out here in the realm remember when the M-60 MG was standard issue, with the utterly inferior barrel change system and re-zeroing requirements, and the MG-42’s barrel change seemed like magic by comparison. Of course, even the M-60 seemed flexible compared to the older Browning 1919 options when barrel burnout occurred! Folks, this Windham rifle shows that the industry & technology have come a LONG, LONG way!

    From a standpoint of basic practicality, this concept WORKS. I’m a retired Army officer, lifelong shooter, and have lived in relatively rural environments where I could hunt in the “backyard,” in suburban subdivisions, and in housing areas/apartment complexes South, West, North, East, and overseas. I first fired the M16 in high school JROTC, owned my first AR when it was SP or nothing at all and this poor college kid couldn’t afford ammo for it (and was viewed as an exotic due to ownership of “military technology”), and lived in the era when 7.62×39 was a Warsaw Pact caliber and .300 Blackout was unborn. Ammo options were simply FMJ, and availability was sparse. Bulk? Forget about it. My point? We’ve come a long, long way in both military and civilian applications of the AR, and Windham’s contribution opens up new vistas that were purely theoretical and futuristic just 20-30 years ago.

    My congratulations and thanks to all involved! And no, I’m not “connected” and am not shilling for anybody or anything.

    • Gibson January 2, 2017, 3:54 pm

      First I want to say, thank you for your service. Second, this article seems to be giving credit to Windham for the innovative design, and doesn’t mention MGI, who came up with this concept and is still making it. MGI calls it the Hydra, and the have a pistol version called the Vipera. Guns America did an article on the MGI Hydra several years ago. I have an MGI Hydra that I bought in 2012. I don’t know if there are any small differences between the Windham and MGI designs, I just want to make sure MGI gets their credit. The pictures of the lower, the magwells and the handgaurd, in this article, look the same as my MGI, in design and function.

      • LouisianaMan January 2, 2017, 6:33 pm

        Thanks for your comment and insights, which show a much greater awareness for modern tech developments than I possess! Sounds like MG1 paved the way and Windham is expanding the application of the approach. I am happy for credit to go to whomever it’s properly due.
        With that acknowledged, as an end user of these advances, I’m just happy that we have progressed this far and show few signs of slowing down. I will continue to root for progress in these areas, and am primarily just happy it’s being made. I’m generally aware of the amazing ammunition developments in 5.56mm caliber, although I have used only M193 and a couple of magazines’ worth of M16A2 with M855. It is encouraging to see how many options and specialized applications keep sprouting from those humble beginnings!
        My thanks to one and all who have helped push that R&D, both on the user side and the industry side.

  • Mike Larsen January 2, 2017, 5:02 am

    I change the stock furniture on all my ARs, starting with the handguard. The one on this thing makes me want to scream, but alas, it appears it can’t be changed since it’s a key, proprietary component of the barrel change system. Hokey handguards seem to be a universal affliction on Windham firearms, but at least they’re replaceable on other models.

    • Ranger Rick January 2, 2017, 10:39 am

      I agree, I’ve handled this system a few times at the Shot Show, while impressive in it’s flexibility the now old fashioned Picatinny carbine handguard needs to go. The MSRP also seems just a bit precious for what you are actually getting. But give Windham they’re due for producing a quality mechanical product.

  • Joe January 2, 2017, 4:32 am

    Colt had a better idea with the 5,56 / .308 interchangeable combo a year or two ago although I’ve not heard much more about it since.
    But my two carbines, one in 5.56 and the other in ,308, both together less than two grand serves me just fine.

  • Will Drider December 31, 2016, 10:42 pm

    Up front I’m not a fan of erector set rifle(s) kits for multi calibers. I don’t like mandated caliber sets, unever wear between key and changeable components and you can’t pass rifle “parts” and have someone cover a tactical position with them. $3K gets you one complete gun and 3 alternative configurations or you could by four $750 stand alone rifles, carbines or AR pistols.
    50 Yard accuracy test was a poor choice. If the 9mm is felt lacking even with the higher velocity created by the longer barrel. Do it a a closer distance. The other three calibers should have been shot @100. After reviewing the results from the 5.56, 300 BO and 7.62X39 groups: they were also poor FOR 50 YARDS! The term “Jack of all Trades Master of none” appears fitting.
    Quality components need to be judged not only on the parts themselves (okay), functionality (okay) and accuracy (not very good). As is, multiple “Match”/caliber change usage would be a stretch IMHO.

    • JC January 2, 2017, 9:35 am

      If you read carefully it appears the author did shoot the 3 calibers at 100 then brought it in to 50 yards for the 9mm. Although who knows since this entire “article” is full of hard to follow statements.

  • Chris Sauer December 31, 2016, 9:12 pm

    Why have you not said 5.56/.223 instead of individually charting both and saying both but all at different times. It sounds like a liberal/democrat is talking about the rifle.

    • Pseudo January 2, 2017, 7:55 am

      I agree it starts with .223 and then goes to 5.56 for testing. I have no desire to get into engineering details, but I have read often enough warnings from manufactures about not shooting 5.56 in guns which specifically say they are .223’s. Basically from I have read these warnings are based on chamber pressures. So what does the manufacture here say .223, 5.56 or both as being acceptable?

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