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Polymer 80% Lower Reality

Filed in AR-15, Articles by on April 19, 2017 0 Comments • views: 183

Everyone by now has heard about 80 percent AR lower receivers.  And more than one company offers 80% lowers made from polymer instead of aluminum. The question is: Can someone without a machine shop or precision milling machine actually turn one of them into a functional and reliable AR-15 lower receiver.  I decided to find out.

I decided to purchase a lower constructed of polymer as well as a low-end drill press and vise. After doing the necessary milling work, I would complete the lower using a DPMS Lower Completion Kit.  For comparison, I would also use a second DPMS Lower Completion Kit to complete an Anderson Manufacturing forged 7075-T6 aluminum stripped lower.

I purchased an 80% lower from Polymer80.com. It comes with the jig as well as the necessary drill bits and end mill bit.  The Polymer80.com website also hosts this great instructional video.

I also purchased the following:

Initial Setup

The first stumbling block was that the maximum dimension between the chuck and the drill press’ worktable was too short: I clamped the Polymer80 lower receiver (and its jig) into the vise and placed the vise on the drill press worktable.  The chuck and drill bit could not be raised high enough to get the vise positioned properly.

Fortunately, the drill press’ worktable was easily removed.  I did so and placed the vise directly on the base of the drill press.  Now I could get the vise and receiver/jig positioned directly under  the chuck with room to spare.  Too much room, in fact. The chuck could not be lowered far enough to drill deep enough into the receiver. Argh.

Careful measurement showed that the base of the vise needed to be raised 1.5″ to 2″.  A quick search on eBay produced people selling aluminum blocks that would work.  I purchased a 5″ x 8″ block 1.5″ thick.  When the aluminum block placed between the vise and the base of the drill press, I had something that would work. (See adjacent illustration.)

Finishing the Polymer Stripped Lower

The actual work was pretty simple – I just followed the directions in Polymer80’s video. I quickly learned that neither my drill press nor my vise are built to tight tolerances. There was a lot of lateral play in the drill press spindle.  There was also a lot of play in all three axis of the vise. I finished the work on the lower receiver with fingers crossed.  The end result is pictured below.  I had my doubts.

For a guy who has built an entire airplane (http://davebarnhart.com/RV6/) I thought the end result was pretty ugly.  The ugliness however did not appear to be such that it would affect functionality and operation of the lower receiver.  We shall see.

The next step was side-by-side completion of the Polymer80 lower and an Anderson lower.

Magazine Catch

Installation of the magazine catch on the Polymer80 lower was pretty straightforward because the openings for it are already milled or molded into the lower.  Assembly was just as easy as on the Anderson lower.

I noticed however that a magazine is a snug fit in the Polymer 80 magazine well. Push the mag release button on the Anderson lower and the mag drops right now.  It would not however drop out of the Polymer80 lower when I pushed the mag release button.  I had to physically grab the mag and pull it out.

The second thing I noticed was that operation of the mag release (as well as the safety) on the polymer80 lower had a rough gritty feel.  I think that is just due to the polymer material.

Bolt Catch

The bolt catch spring and plunger installation is identical for both receivers. The hole is already milled/molded for it on the polymer80 lower.  However, instead of using the normal roll pin to secure the bolt catch, Polymer80 provides you with a tiny little pin which you drop into the hole.  The pin is then secured by screwing a tiny allen screw into the hole. You can see the allen screw in the lower portion of the photo to the right.

Note that the hole is not threaded. You must screw the allen screw in with some pressure and self-tap the hole. It was not easy to apply sufficient pressure and hold the alen wrench straight at the same time.

After installation the bolt catch seemed to work just as well on the Polymer80 lower as it did on the Anderson lower.

Pivot Pin

Installation of the pivot pin was identical on both lowers.  This job is made a lot easier if you have the Brownell’s Pivot Pin Installation Tool.

Safety Lever & Pistol Grip

The safety lever slid right into place on the Anderson lower. On the Polymer80 lower I needed to deburr the holes a bit. As with the magazine catch, operation of the safety lever on the polymer80 lower had a rough gritty feel.  I think that is just due to the polymer material.

You’ll notice from the adjacent photo that the Polymer80 lower is not threaded for the pistol grip screw.  Instead, they furnish a square nut that slips into a slot. The 3/4-inch screw supplied with the Polymer80 lower to fasten the pistol grip to the lower was not long enough. I needed to swap it out for a 1-inch screw from Ace Hardware.

Trigger Group

Installation of the trigger group of both lowers was identical.  I did need to slightly deburr the holes for the pins.

Trigger Guard

The Polymer80 trigger guard is molded into the receiver.  You can throw the trigger guard and its roll pin into your ‘spare parts’ bin.

Buttstock

I installed Brownell’s M4 Stock Assemblies on both lowers.  As expected, assembly on the Anderson lower went without a hitch.

On the Polymer80 lower, the hole for the buffer retainer needed to be deburred in order for the buffer retainer to fit and move smoothly.

Problem 2:As you can see in the adjacent photo, the rear takedown pin does not protrude all the way through and extend above the surface of the lower.  This means it’s not going to be possible to push the takedown pin out with using some sort of tool.  Also, the holes in the receiver for the takedown pin are oversized. SInce these holes were factory-molded into the receiver, I cannot blame my cheap drillpress and vise.  Fortunately, there is no worry about the rear takedown pin accidentally falling out due to the takedown pin detent and spring, but that revealed the third problem:

The first time you removed an AR-15 buttstock, the takedown pin and spring may have taken you by surprise.  There is a pin – backed by a spring – that rides in in a hole in the receiver. The nose of the pin runs in the groove in the rear takedown pin. The aft end of the spring is held in place by the receiver end plate. (In the adjacent photo, the yellow line illustrates the position of this tiny pin.) In the Polymer80 lower, this hole needed deburred in order for the tiny pin to move freely and do it’s job.

Mating Season

With both lowers complete it was time to mate them to an upper and do function checks. For this, I used an Anderson complete upper, and it mated to the Anderson lower with no problems.  The result is a nice light smooth gun.

Mating the Anderson upper to the Polymer80 lower was a different story. If you examine the adjacent photo closely the holes in the upper and lower for the (rear) takedown pin are misaligned.The holes in the Polymer80 lower are too far aft buy about 10-20 thousands of an inch.  As a sanity check, I grabbed the upper from my Windham Weaponry SRC and tried to mate it to the Polymer80 lower as well.  The misalignment was even worse. The holes for the takedown pin and for the (front) pivot pin are predrilled at the factory, so I cannot blame this on my lousy drill press.  The best I could do at this point was to use my Dremel to elongate the holes in the Polymer80 lower so it could mate to my upper.  Very ugly but it worked.

The Proof of the Pudding is in the Shooting

It was time for a trip to the range.  The short version of the story is that both lowers functioned perfectly at the range.  I put 100 rounds through each one.

Summary

As you can probably tell, I was not happy with the Polymer80 lower. I want to have pride in my guns.  When I hand one to a friend I want to be proud of it. There are enough defects in the Polymer80 lower (particularly the takedown pin holes) that I certainly cannot hand it to a friend with pride.

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