March 07, 2016
By Steven Matthews
Traditional gunmaking materials like steel and aluminum are more and more being replaced by modern polymers. Many of today's plastic wonder guns run almost flawlessly, so I guess you can't argue with success. Even though I'm not a huge fan of polymer guns, I do have several in my collection that function to near-perfection, and we'll take a look at building a polymer 80 percent lower receiver for an AR-15.
Besides a couple Glocks and Kahrs, I also have an AR-15 in .22 Long Rifle that I built from parts that features both a polymer upper and lower receiver. It has fired thousands of rounds with very few malfunctions. Besides functioning great, it also is extremely light at only 4.5 pounds.
Today polymer has a strong foothold in the AR-15 market, with several brands being made that feature polymer lower receivers. Some even feature polymer uppers. The jury is still out on whether this is a good idea or not, as it will take years of use to determine if AR-15 guns built from a polymer 80 percent lower receiver will hold up for decades like the metal versions have.
I have seen pictures on the internet of polymer AR-15s made from 80 percent lower receivers that have failed dramatically by breaking into many pieces. Conversely I have also read reports from polymer AR-15 owners who report thousands of rounds fired with no problems at all. While there are dozens of aluminum 80 percent lower receivers on the market, there are only a few polymer 80 percent lower AR receivers available to the hobbyist.
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An internet search led me to a company called Polymer 80. Polymer 80 is a new company that has just started making a polymer AR-15 80 percent lower receiver.
Several conversations I had with the owner, David Borges, convinced me that they were very interested in supplying a quality product to the gun-building hobbyist. The rollout date for the product was delayed several times until the product met his quality standards. Rather than rushing the product to market to cash in during the recent buying panic, Polymer 80 waited till the product met their standards before releasing it to the public.
The Polymer 80 percent lower receiver features unusual styling. Injection-molded polymer allows for designs that would be cost-prohibitive to forge in aluminum.
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Finally in late October of 2013, they began shipping product out to customers. It appears to have been worth the wait. Space doesn't allow me to make a big sales pitch here so I will just refer readers to the Polymer 80 website. There you'll find in-depth information on the product.
There are many other 80 percent lower receivers I saw online. I have no financial interest in promoting this particular product; I just am using it to show my readers that they can complete an 80 percent lower receiver by themselves. With that said, let's get back to reviewing how to build a polymer 80 percent lower receiver.
The integral trigger guard on the Polymer 80 receiver increases rigidity of the receiver. The trigger slot is filled with polymer that will be removed later.
The Polymer 80 percent lower receiver offering is brand new to the AR-15 market and is made from what they are calling a proprietary polymer especially suited for an AR-15 receiver. The base price of the Polymer 80 percent lower receiver was, as you might guess, $80. Upgrades in the way of colors and graphics are available at additional expense.
My basic black receiver featured a finely textured finish so that it did not look like cheap shiny plastic. As usual, you'll have to machine out the cavities for the fire control components and drill the holes for the hammer pin, trigger pin and safety lever. The Polymer 80 percent lower receiver is injection-molded and you can see a very fine line between the mold halves.
The portion of the 80 percent lower receiver that needs to be machined to create the cavities for the fire control group is colored red so you can use the color as a guide for removing material. Material removal can also be determined by measuring or by using the Polymer 80 percent lower completion fixture.
The Polymer 80 magazine well is somewhat abbreviated in depth when compared to a GI AR receiver. The maker claims this eases magazine changes.
The Polymer 80 percent lower receiver is styled differently from a GI-format AR-15 receiver. Most noticeable is the styling of the magazine well and the integral trigger guard. The receiver also has reinforcing ribs molded into the upper sides of the receiver.
The front pivot pin area and the buffer tube ring have been made thicker than an aluminum receiver. One much appreciated modification concerns the grip screw. Rather than having very fine threads formed in soft polymer, this 80 percent lower receiver has a small steel nut that fits in a pocket. The steel grip screw engages the steel nut, thereby reducing the risk of stripping soft polymer threads by even the slightest bit of over-tightening.
When you pick up a polymer AR-15 80 percent lower receiver, you immediately notice, its main benefit: it's as light as a feather. Once machined, my Polymer 80 percent lower receiver weighed in at 110 grams (about 3.5 ounces) versus about 260 grams (about 8.5 ounces) for a GI-format aluminum receiver.
The receiver features reinforcing ribs that extend all the way across the top of the receiver, while the gussets around the pin holes are made thicker for strength.
To determine the correct places to machine the cavities and drill the holes, Polymer 80 offers a well-designed completion jig. This jig, which includes tooling, is priced at only $20. I don't know how Polymer 80 can sell this fixture for only $20 with tooling (three drill bits, an end mill, and a stop collar)!
The jig itself consists of two pieces of injection-molded plastic that completely surround the receiver. The sides of the jig feature partially drilled holes that provide for precise location of the hammer pin, trigger pin, and safety lever holes.
The top of the fixture features five holes to begin the process of forming the cavities for the fire control group. Later, I'll briefly cover the processes used to complete the cavities. It's not readily apparent how it is used to form the cavities. That is because this jig is a sacrificial fixture. During the machining process, part of the fixture is machined away. What you see at the beginning is not what you see at the end.
Since part of the jig is sacrificed, it should be considered a single-use product, although some of the fixture could be re-used on other projects. Considering the low $20 cost, the single use issue shouldn't be a big concern.
The buffer tube ring is thicker, but don't think you can crank the tube in recklessly. Use thread locker to limit the torque required when installing the buffer tube.
The Polymer 80 percent lower receiver and fixture are very easy to use once you know how they work. Very well written and in-depth instructions are available through the Polymer 80 website. The instructions list three different methods to complete one of the Polymer 80 percent lower receivers.
It does have some areas where there is extra material but it looks to be just a feature of the molding process, rather than an intentional reinforcement. The trigger guard is integral, so that would count as reinforcement versus a GI-format trigger guard.
The Polymer 80 percent lower receiver can be completed using multiple methods. You can machine it accurately or crudely carve it out like a birdhouse! Even the crudest method can achieve good results if you take pride in the work and pay attention to detail.
Keep in mind that once completed, the gun is a $500+ firearm and your receiver should be as good as the rest of the project. While looks are important here, the most important issue is that the gun is safe and that it functions properly.
Rather than having very fine and weak polymer threads for the grip screw, the receiver features a pocket that accepts a steel insert that secures the screw.
While the gun can be made with crude looks and still function fine, you should strive for both good looks and fine functioning and safety.
I want my projects to look and work just as well as any factory-made gun. If you do good work, this project can be indistinguishable from a factory made firearm. I urge you to put the effort into this project that it deserves. In this article I will explain how I completed this project receiver. If one does not like my methods then by all means make your project the way you want, it's your project.
The polymer 80 percent lower receiver company recommends three different methods in its instructions, using a milling machine, using a drill press and finally by using basic hand tools.
Call me lazy if you want, but I chose to use the milling machine and drill press methods. The hand tool method will work but it is extremely labor intensive. I did the first receiver with the drill press method. The Polymer 80 percent lower receiver instructions available through their website are fairly complete (12+ pages) so I will just give readers a quick overview of the process here.
First place the 80 percent lower receiver into the jig segments and use the supplied screws to secure the sections together. The fixture is then placed on the table of the drill press. Make absolutely sure that the table is precisely square to the quill and drill bit.
Drill six holes through the fixture and into the sides of the receiver. Four holes will be 5/32" and two will be 3/8". While Polymer 80 percent lower receiver company recommends drilling these holes full size with the supplied drill bits, I prefer to drill them one size under and then use precision reamers to bring them up to full diameter for best accuracy.
Unlike a completed AR-15 receiver (c.), 80 percent lower receivers require that the hobbyist builder finish machining out the cavity for the fire control group.
Drill bits are not designed to form precision holes; they are better thought of as general-purpose hole formers. Drill bits many times drill holes a few thousandths over or under their stated sizes. I prefer my holes to be exactly on size for the best parts fit.
When drilling your holes, only drill about halfway through the 80 percent lower receiver. Do not try to drill all the way through from one side. Doing so will almost certainly result in holes being off location on the opposite side.
Once the 80 percent lower receiver holes are done, it is time to begin forming the cavities. To get the cavities right, you will need to do some fairly accurate measuring. Set a depth collar on the supplied end mill so that when used with the jig, it will cut a cavity to the specified depth. Setting the collar to the depth listed in my instructions would not have resulted in the correct depth on my receiver.
Fortunately I checked before I attempted to form my cavities. Apparently there was either a typographical error in the instructions or there had been a design change somewhere that altered the results. The cavities are supposed to be 1.25" and .630" deep, so I had to set my collar to a dimension that created this depth, not the dimension listed in the instructions.
Before cutting your cavities, verify that the setting you use on your collar will result in the correct depth. Once you have the right dimension, the fixture will create the right depth in each cavity when the mill is pushed in till the collar stops it.
The Polymer 80 completion fixture features holes in the sides and top that will be used as guides for machining. Drill sizes are molded into the fixture.
With the collar set to the right dimension, you can get on to cutting the holes in the top of the jig. You need to set the jig and receiver assembly up squarely in a cross-feed vise, and that vise in turn has to be securely clamped squarely upon the drill press table. If you don't have everything square, your 80 percent lower receiver walls will not be parallel because the mill will cut at an angle as it is being fed into the work.
In the most extreme case of not being set up square, you could end up with an 80 percent lower receiver wall the right thickness at the top but have the mill cut right through the side of the 80 percent lower receiver wall at the bottom.
I will be the first one to say that this really isn't the right way to machine a receiver. A drill press is not designed to be a milling machine. This method is an extremely improvised way of making a receiver.
This method is not my method, so don't squawk to me about using improper machining methods. This is the way that is recommended in the supplied instructions.
The first operation will be to drill holes to form the trigger slot. The instructions say to use a 5/16" drill for this job since that is the finished width of the slot but I used a smaller 1/4" drill so that I could hand file the slot to exact size later.
Now it is time to start cutting out the cavities. The end mill that you set up with a stop collar will allow you to feed it into the jig and receiver and end up with a flat bottom on the cavity. Set the end mill up in the drill chuck and then plunge it down into the receiver and jig.
Again I must say that I know this isn't the recommended way of doing this job, but this is the way the instructions say to do it. The end mill is fed down into several holes in the top of the jig to create holes that are the correct depth.
Then move the jig slightly and run the mill down into the jig again. This will create oval holes that will be the front and back of the cavities. You will be making four oval holes and one round hole with the mill. Be absolutely sure you have done this portion of the job right before moving on.
The receiver fits within the fixture halves and is securely held captive once the two screws are installed. This fixture is a bargain at just $20, Matthews says.
To allow for easy access and viewing of your work area, cut off the top of the jig at the line marked on it. I cut mine off with a hacksaw, but you could also just mill it off since you have the jig set up in a cross-feed vise.
Now the rest of the cavity is cut. The front cavity will be 1.25 inches deep and the rear will be .630" deep. You should try to stay within about .010" of these dimensions. You will need to set the depth stops on your drill press to keep the mill cutting at the same depth as the previously formed holes.
You have two options for removing the rest of the material of the cavities. You can cut it out by plunging the mill down into the work and then pulling it up and moving the work slightly and then plunging down again. You can do this over and over till you have the entire colored insert removed. The rough edges can be smoothed up by sanding or filing.
The other way is to plunge the mill down to the right depth and then slowly move the workpiece with the cross-feed vise to cut out the material as you would with a milling machine. This will give a smooth wall that will not have to be filed or sanded.
If you have a milling machine, you can cut off the top of the fixture to access the top of the receiver. Then mill out the cavities with an end mill.
Removing all the colored insert is supposed to give you a cavity that is the right size. I prefer to measure my cavities. For those who want to measure, I will give you some rough measurements for the cavities.
The front large cavity should start .100" behind the slot for the bolt hold-open lever. If you go much less than .100", you run the risk of cutting into the magazine catch spring cavity.
The width should be from .690" to .700". The length can just be to the end of the colored insert. The smaller rear cavity is .500" to .510" wide and will run from the front cavity to about .085–.095" from the buffer detent hole.
There is a narrower section above the safety lever hole that is only about .420" wide. If you cut the small cavity any wider than .510", your 80 percent lower receiver fit may be sloppy. After you have the front and rear cavities cut, you'll need to deepen the front portion of the rear cavity so it is the same depth (1.25 inches) as the front cavity. This deeper portion will run from the front cavity to about .100" behind the safety lever hole.
The front and rear ends of the large cavity are cut with the mill being run down through the guide holes. Double cutting will create oval shaped holes.
Once all the cavities have been cut to the correct sizes, deburr the edges and file out the rest of the trigger slot. After my cavities were formed, I installed a fire control group and made sure everything worked OK.
As long as you have the hammer and trigger pin holes spaced right, you should have no problems with fire control group function. You need to verify that your disconnector functions correctly and that the trigger engages the hammer correctly.
The only remaining job is to work on the bolt hold-open device. Rather than using the GI method of using a 3/32" roll pin to secure the BHO lever, the Polymer 80 percent lower receiver uses a short dowel pin in a blind hole that is retained by a small set screw.
The threads for this very small (5–40) screw have to be cut by the builder. I think the difficulty of threading this hole means Polymer 80 percent lower receivers should be done at the factory. The hole is in an awkward place for the hobbyist to thread it.
Since I did not have an extremely small tap extension, I had to turn the tap into the hole very slowly with a pair of miniature needle-nose pliers, not the best way to operate a tap! The set screw doesn't go very far into the hole so only thread it a few turns deep. Use some thread locker on the set screw and do not overtighten the screw or you risk stripping the polymer threads.
While Matthews sees the value of fixtures for the beginning builder, he's experienced enough to prefer to make his own measurements and cut accordingly.
I previously mentioned that I used a different method on the other receiver. My second receiver was done using a milling machine and this made the job immensely easier and faster. While I did use the Polymer 80 percent lower receiver fixture, I used it with my own methods that really sped up the build process.
I have previously machined a dozen other AR-15 80 percent lower receivers, so I was very familiar with this method. Due to the excellent design of the Polymer 80 percent lower reciver completion fixture, it only takes a few minutes to drill and ream the hammer, trigger and safety holes. There is no easier way to make the holes so I just used the same method as before to form the holes.
Now comes the part that makes this an easy job if you are skilled with a milling machine. This method allows you to complete the cavities in less than 30 minutes. First, set the receiver up (mounted in the Polymer 80 percent lower receiver fixture) in your milling machine vise. It must be set up precisely square and aligned with the X and Y axis.
Then either cut off the top of the fixture with a saw or mill it off so that you have the top of the receiver completely open and accessible. I then installed a long 3/8" end mill in the quill. I set the quill depth stop so that I could plunge the mill down into the receiver to a depth of 1.250 inches.
The set screw hole has to be threaded by the builder, and its awkward position makes that difficult. Matthews thinks this job should be done by Polymer 80.
Note here that you don't have to go full depth all at once; you can get to depth in stages if you prefer. The larger forward cavity will now be rough cut to size. For a preliminary rough size cavity, I just stayed well within the edges of the colored insert.
I just plunged the center cutting mill down to depth and ran the mill around the edges to cut out the cavity. Polymer cuts very easy, so this job is quickly completed. I finished the cavity to a precise final size by using measurements.
My cavity was .695" wide by 2.050" long. The cavity is centered between the receiver walls so that you have equal thicknesses on the walls when complete. The front of the cavity was located .100" back from the slot for the bolt hold-open lever.
You need to have at least a .100" thick wall at the front of the cavity to prevent machining into the recess for the magazine catch spring. If you go much under a .080" wall, you will likely have a hole in the front cavity wall. This won't ruin the project but it doesn't look good having an "extra" hole.
The rear cavity was then cut to a depth of .625". It will be .500–.510" wide, except over the safety lever hole, where it will only be .430" wide. The rear cavity runs from the forward cavity to within .075–.085" of the buffer detent hole. The exact size of the portion over the safety hole isn't critical; I just made it a little wider than the 3/8" safety lever hole.
The Polymer 80 receiver uses a dowel pin and set screw to retain the bolt hold-open lever rather than a 3/32" roll pin as utilized in a GI receiver.
At the junction between the front and rear cavities, the narrower rear cavity will need to be deepened so that is the same depth as the forward cavity (1.25 inches). This is for clearance on the rear of the trigger. The deeper portion follows the shape of the rear cavity that you already cut. The deeper portion runs from the front cavity to about .100"–.125" past the safety lever hole. Length is not critical.
All that remains is to cut the trigger slot. If you cut your cavities to the right size, these supplied dimensions will get you close to the right size. I recommend cutting considerably under size then opening the slot up by hand filing to final size. The slot will be .280-.310" wide depending on the width of your specific trigger. I start my slots at a point .660" back from the front receiver wall. The slot will then run to 1.187 inches.
This pretty much covers the milling machine method of cranking out an 80 percent lower receiver. Once you are familiar with the process you should be able to complete a polymer or aluminum 80 percent lower receiver in 20–30 minutes with a milling machine. Of course, you could do this entire cavity cutting by hand. If you don't mind the tedious labor you can cut out the cavities with nothing more than hand tools. Obviously this would take hours compared to 30 minutes with a milling machine.
Overall I was very pleased with my experiences building a polymer 80 percent lower receiver for an AR-15. Compared to aluminum receivers, polymer receivers are very easy to machine. For those using basic hand tools to complete the receivers, this easy machining would be a real plus. While the polymer is not as strong as aluminum, an AR-15 receiver is not highly stressed.
I have an AR-15 in .22 Long Rifle with both an upper and lower polymer receiver and it has held up fine for thousands of rounds fired. Whether a polymer 80 percent lower receiver would hold up this well under the higher stresses of firing the .223 Rem. rounds remains to be seen. At today's high ammo prices I certainly can't afford to do a multi-thousand round endurance test on polymer 80 percent lower receivers.
Internet chatter would indicate that under the right conditions a polymer 80 percent lower receiver AR-15 is OK. Personally I would not ever consider using a polymer AR-15 for self-defense or combat, but I have no problems what so ever about using one on the gun range for recreational purposes.
Will a polymer 80 percent lower receiver AR-15 last for decades like an aluminum one will? Well only time will tell on this issue, they haven't been around long enough to establish a long term history. I am however going to own and shoot my polymer AR-15 guns for years to find out if they will make it in the long run. From my limited experience with polymer receivers I think they will hold up fine for thousands of rounds if they are built and maintained correctly.
Who knows, I never thought polymer handguns like Glocks would ever turn out to be great guns. Perhaps 20 years from now we will look back and wonder why it took so long to use polymer on the decades old AR-15 design. If completing your own 80 percent lower receiver, weather polymer or aluminum, sounds interesting to you why not give it a try?
