One of the more interesting phenomena to watch the last two presidential elections has been the explosive growth within the AR15 80% lower receiver market, and today we’re going to build one from Polymer80.com.
Why make your own AR?
An AR-15 is a modular beast. Though more than 200 companies sell specific configurations, many shooters prefer to put their own rifle together from a variety of modular components. They may buy a complete rifle, but shortly after they finish adding things to it, they begin swapping parts out.
But there’s more. The lower receiver is a registered part. When you buy an AR-15, the piece that makes it a firearm is the lower, which will have a serial number that is recorded on the 4473 that you fill out with your local FFL. Even though we aren’t supposed to have a national gun registry, many are hesitant about telling Big Bother that they own a scary black rifle. The DIY lower is a way around this. Any hunk of aluminum, steel, or even polymer could be built into a lower, if you had the technical know-how and the right tools. And if you do this yourself, and you do it for yourself, you are not required to tell anyone, fill out any forms, or register anything. If you make a gun for yourself, one you don’t intend to sell, it is perfectly legal. That’s the real appeal here. Make yourself an AR, and do it yourself, and no paperwork.
This is perfectly legal as long as you are “legally able to purchase a firearm” and do not sell them without first registering the “made” receiver. If you do want to sell a home brew firearm receiver, you will need to register it, which requires a $200 tax stamp registration, which is hardly worth the cost (unless you created one seriously unique masterpiece).
Some companies have made this whole process easier by creating kits that take an almost-completed-receiver-like-thing and some instructions on how to remove the rest of the superfluous material. These are commonly referred to as 80% receivers. They are 80% complete. Remove the extra 20%, and what started off as an almost-receiver suddenly becomes (legally and functionally) a real firearm. Some of these kits are aluminum, some polymer. And some are easier to finish than others. While a polymer kit (with a good jig–like the one we’ll look at below) can be finished with a simple drill press, the aluminum ones are more complex. Most mill those, and that’s not a skill most of us possess. There is the temptation for some to buy the kit, and someone else finish it (like a qualified machinist). At this point in time, the rules on that are really murky–it is best to do it yourself. And far more gratifying. Finishing an 80% lower allows a home hobbyist to make a firearm at home for the experience and pride of a job well done.
For the vast majority of us though, we just love the idea that as a law abiding gun owner can legally make and shoot a gun no one knows about or is tracking. Yeah, that whole 2nd Amendment rights thing. The general idea of an unfinished 80% lower receiver is that a buyer can use a partially complete 80% AR15 receiver to make their firearm. As far as the ATF is concerned, 80% lower receivers are still just a chunk of raw material and are not considered firearms. As such, an 80% lower receiver can be easily purchased without an FFL transfer just like any chunk of aluminum or polymer. Heck, they can even be legally sold in vending machines, or to minors since they are just raw materials. Wouldn’t that freak out the anti gunners?
The build process
I will note that DIY home firearm building is not the least expensive or easiest route to building a firearm. Picking up an inexpensive finished cosmetic blemished $50 receiver from your local FFL dealer is the easiest, however that is not what making your own gun is about. The basic polymer lower and jig comes in at $95. But like building your own guitar, or building a classic car, it isn’t about the cost. This isn’t something you do to save time or money.
By ATF’s definition, an AR15 lower receiver is not considered a firearm until it has the ability to accept a fire controls (trigger, sear, springs, …etc.) Until the firearm reaches that state, it is not considered a firearm and therefore is not a regulate-able firearm. Using this ATF ruling, most 80% lower receivers have all the more complex machining aspects of the lower receiver complete except the fire control pocket (where are the trigger control parts are installed) and trigger, hammer and selector switch holes.
The 80% terminology is also a little misleading as I would argue that milling out a fire control pocket and drilling a couple holes does not represent 20% of the work, however the 80% name stuck and that is what the product is referred to industry wide. Some manufacturers claim to have 90% lowers, others sell 80% lowers which are more like 50% complete and require extensively more machining than most industry standard models. Some manufacturers offer completely raw AR15 forging receiver blanks for the super adventurous home fabricator. There really is no standard when it comes to 80% lowers.
Polymer80.com has come up with an interesting concept with a polymer AR15 80% lower receiver kit. The kit delivers an easier path to finish the drilling and milling than regular 7075 or 6061 forged aluminum receivers for DIY home gun makers. The reason is mainly due the softer/easier to cut polymer construction. The Polymer80.com has most of the key areas pre-molded/machined, includes a disposable jig, and even drill/mill bits in a kit all for around $95. It is about the same price as a decent finished aluminum forged receiver, however it includes everything needed in the kit to complete the finishing excluding power tools.
Making your own AR15 lower receiver from an 80%’er usually requires creating a fire control pocket and drilling three holes: two for the trigger and hammer pins, and one for the selector switch. These are tasks which are all easier when the material is polymer versus forged aluminum.
Drilling the three holes is something anyone can do assuming they know where to drill, however creating the pocket is generally where the challenge begins. One of the huge appeals on the Polymer80.com lower is that you can just drill and then carve the fire control pocket out with a Dremel with just a light amount of cursing. This drill and Dremel finishing technique is theoretically possible with aluminum lowers as well, however very painfully slow and generates far more cursing.
Most 80% receiver manufacturers also offer some type of drill jig which provides guides for drilling the holes and supporting various methods creating the fire control pocket. Bolt the jig to the 80% lower, drill the holes and you at least have places for all the pins and selector switch to go and potentially some of the key areas milled out on the fire control pocket. The fire control pocket is the more difficult machining operation especially with very hard 7075 aluminum forgings, however many companies have figured out ways to create pockets more easily with milling guides and drill-n-Dremel templates.
Polymer80.com saw this challenge and decided to put together a kit for the DIY’er who probably already owns either a free-standing or tabletop drill press. If you do not, Harbor Freight has tabletop drill press models regularly available for around $70. An additional drill vise is a requirement and preferably one which is an X-Y Axis machinist vise for the drill press. What this drill press and vise will deliver is a vertical precision cutting capabilities of an end mill to perform move and drill material removal. It will not deliver the precision X-Y milling capabilities of a mill, however it will get you very close so that the final fitting and Dremel work is not so extensive and time consuming.
Polymer80’s jig is based on drilling out the majority of the areas with the included mill bit and drill press and then using a Dremel tool to finish things up. More specifically, the fire control pocket guides combined with the triple flute mill and set depth gauge allow all the key front and rear fire control pocket areas to be cut to the right depth and position. All that remains is to just remove everything in between the cuts at the same depth.
Included in the kit is a 5/32″ drill bit for the two holes for the trigger and hammer pins, 5/16″ bit for the trigger shoe hole, and 5/8″ bit for the selector switch. Also included is a 7/16″ triple flute end mill to be used in the drill press to mill out the rough fire control pocket.
The inclusion of the end mill in the Polmer80.com is a plus as typically the bulk removal process is accomplished via a 3/8″ or 1/2″ drill bit. The pointed drill bit tips do not deliver a clean flat cut on the bottom of the pocket and don’t work particularly well moving sideways through material if you do have that capability with your drill press; an end mill bit does a great job at all these things. I have done the drill-n-Dremel method and my before and it worked, however I would rather use an end mill bit which will deliver both a far superior finish and a faster completed product.
Most 80% Jigs provide End Mill cutting guides. A smaller tabletop end mill like the $880 3960 HiTorque Mini Mill I own from Little Machine Shop is awesome and can do anything required for making your own lower receiver. Now that I own this wonderful and fabulous tool, I will never return to the drilling and Dremeling method of finishing a 80% lower. This mill does a great job on aluminum, however on polymer it is stunningly fast. I was able to finish one of the Polymer80.com lowers in about 30 minutes start to finish. The only real time consuming items were the initial setup and managing all the spiral streamers the end mill bit sheds during the cutting process.
Included in the bag is the polymer lower receiver, the drill bits, the jig, set screw bolt catch, pistol grip nut, pistol grip screw, Allen wrench, and jig screws. Slip the jig around the lower and screw in place with the included screws. The trigger and hammer pin holes can be drilled with the 5/32″ bit and the selector switch hole with the 3/8″ bit using the the jig’s drill guides. Obviously a drill press is highly desirable even for this step to deliver a 90 degree hole perpendicular to assure proper trigger, hammer and selector switch alignment and functioning. The milling out the fire control pocket may seem like the chore, however drilling out these three fir control pin holes are actually the most critical from a precision perspective. The pocket milling can be quite a bit out of spec and still deliver a perfect functioning rifle.
Standing up the jig, the jig provides guide holes for drilling out the trigger shoe hole with the 5/16″ bit and straight cut holes for the mill bit to remove larger portions of the trigger control group and provide start and stop points for milling. Polymer80.com supplies a stop collar for the end mill bit which is very helpful whether you opt for a drill and Dremel route or own a mill. Using the preset stop collar and end mill bit you can drill start and stop points to the correct depth for the upper and lower stepped down areas. Just tighten the mill in your drill press and drill down until the collar hit the top of the jig at two front and rear points for the upper pocket and two front and rear points for the lower fire control pocket areas. Probably one of the easiest kits I have found simple due to this very elegantly delivered feature of the kit. If you have a mill as I do, these guides coupled with the included end mill and stop collar provide very quick reference points.
At this point the top of the jig must be cut or milled off to expose the remaining milling/Dremel jig guide-lines. If you do not have a mill, your drill press can be used with the end mill bit to move from area to area within the guide to just drill out the bulk of the material and then finish and fit the trigger control with Dremel “tuning”.
I used my drill press for the drilling and then moved over to my Little Machine Shop HiTorque Mini Mill vice. Once mounted, I used the front and rear jig guides to start the pocket milling and then milled off the top of the jig guide. Once that operation was complete I used the guides on the disposable jig and began milling out the fire control pocket.
A final test fit of the hammer and trigger group and I had a completed lower ready for a final inspection and assembly. With polymer lowers, home builders will find that polymer flexes during the machining process, so holes will actually be a little undersized and the sides of the trigger control pocket may need a little extra tuning after being freed from the jig. Final finish steps are to clean out all the pre-drilled holes including detent holes and hit the sharp edges with a little sandpaper and you are ready for final assembly.
Polymer80.com has included a couple hardware pieces which should be used during assembly to increase the strength and durability of the polymer lower. These pieces include the set screw bolt catch pin, dowel pin for bolt catch, pistol grip nut and grip screw.
Most people will just use whatever the cheapest parts kit they can find however I think that is the wrong way to go on a AR1 lower you will have to the rest of your life. I used a high quality Barnes Precision Machine AR15 parts kit for the assembly of the lower plus a very nice billet PWS premium buffer tube assembly and a Mission First Tactical Minimalist buttstock. The build went together as easily as any other AR15 lower receiver I have assembled.
I was actually so impressed with the looks of this lower that I decided a well appointed upper was in order. The rifle is finished with an Aero Precision Upper, WMD NiBo Carrier, Sharps Relia-bolt, Aero Precision Lightweight 5.56 Barrel, PWS Brake, Parallax Tactical Gen 2 15” Free-Float forend, AB Arms T-Grip, and the Konus Pro optic I have been testing out on builds.
One of the considerations many have of polymer receivers is durability, however I have not had any realistic reliability issues with any polymer receiver I have ever tested including this Polymer80.com version. Now if you want to go and attempt grossly negligent destructive tests, I am positive you can break it, however no normal use I threw at the Polymer80.com produced any unsurvivable blows. Heated arguments can be made on whether well designed Polymer lowers are actually stronger than aluminum version… after all polymer lowers are all the rage. Due to the flex of the polymer, I would bet that a well designed polymer lower might actually take more abuse because it will flex instead of break, however that is an argument I have yet to put to the test.
I will skip that highly contested point and just note that I had zero issues with any pins coming loose, no stress fractures, or performance related issues after over 550 rounds of 5.56 NATO spec ammo. All the typical magazines from metal Mil-Spec, Magpul, and Troy all worked perfectly and dropped free from the receiver.
The profile/molding of the Polymer80.com is much beefier in the higher stress magwell and buffer tube union areas than a MilSpec profile forging which also delivers increased strength and a cool custom look. From a looks perspective, I give the Polymer80.com receivers high marks as passing as a custom billet receiver especially after dressing it up a bit with a few cool accessories.
One big advantage to the molded in color is that scratches did not show like they would on a black anodized aluminum receiver.
I have finished more than a few 80% lowers; some forged 7075 aluminum, a couple in billet aluminum, and a few in polymer. The Polymer80.com was far easier even on the mill to finish up than anything in aluminum and for the guy with limited machining capabilities, the polymer material is definitely the way to go especially for a full kit priced under $100.
Part Description ITEM Qty per Lower
- AR15 Lower Receiver G150 1
- Set Screw Bolt Catch Pin 3mm 1
- Dowel Pin for Bolt Catch 5/8in 1
- Pistol Grip Nut 3/8in 1
- Pistol Grip Screw 1.5in 1
- Allen Wrench 1/16th 1
- AR15 Jig J150 1
- End Mill and Stop Collar for Fire Control Pocket 7/16” end mill 1
- Drill Bit, Trigger Shoe Hole 5/16” drill bit 1
- Drill Bit, Safety Selector Switch Hole 3/8” drill bit 1
- Drill Bit, Pin Holes for trigger and hammer 5/32” drill bit 1
- Screws for tightening Jig 6
- MSRP $160.99- Polymer80.com