Apex Gunsmithing
9Apr/14

Poly Receivers Testing: Part One

With the recent break in the weather, things are finally getting done around here!  We're still backlogged beyond belief and have a lot of catching up to do, but in between major undertakings I've managed to grab some images and do a little testing!  By testing I mean beating on it.  A lot.

Test Rifle Overview

DISCLAIMER: Receiver provided by TN Arms Co for testing purposes.

Here's the rundown on the build itself:

  • New Frontier Armory Accu-Group polymer lower parts kit
  • Palmetto PTAC Midlength upper (thanks to M.H.!)
  • Magpul K2 grip, Palmetto stock kit with plain M4 stock
  • Mepro21 (thanks to J.A.!) only used for live fire
  • Pictured with CMMG Stainless Evo .22LR conversion unit, which I am also reviewing
  • Rifle weight w/o optic: 6lb 6oz

TN Arms FDE Test Lower

Highly Scientific Testing Process

In the past, polymer receivers have failed in a pretty predictable and spectacular fashion by snapping clean off at the buffer tube area.  The stress point seems to lie right where the rear takedown pin hole sits, so TN Arms has reinforced this area significantly on top of the molded in brass collar.  The latter component solves the issue of stripping out plastic threads using the aluminum buffer tube.

My first test was to chuck the lower in a vise and see just how well that molded in bushing holds up.  Short answer: exceedingly well.  Long answer: without a cheater bar and short of doing full on chin-ups off of your armorer's wrench, I can't see any way to damage this thing.  The guys at TN Arms did this test and suggested it to me, and my findings are identical.  No worries at all in this area.

The next logical step is to find a way to stress the buffer tower area in different directions, to ensure that there's no chance of the stock snapping clean off as other receivers have in the past.  My testing is focused on practical, real world beatings that you could potentially encounter when using the rifle yourself, so the first battery of testing is as follows:

  1. Induce stuck case, mortar.  Repeat 20-30 times.
  2. Apply blunt force from both sides and the top at the stock.
  3. Tread on empty magwell

I'm open to suggestions for additional ways to torture test the lower!  I don't believe live fire is sufficient unless someone loans me a .50 Beowulf, so physical beatings are the order of the day.  Simply put, number two on that list means beating it against a tree or something just to apply a lot of torque and force to the buffer tube area.  The stock and tube belong to me so I'm not afraid to whack it around!

FDE Lower Closeup

More Thoughts on Quality

The image above shows some of the rough edges present on the TN Arms lowers.  I've now seen six of them and they've gotten better since the first batch, so I'll give them a lot of credit for constantly improving their product.  TN Arms also offered a trade-in program to get their newest generation lowers out to early adopters who bought the earlier runs, even if there were no issues present.

The first few receivers I got in had mold defects all over the place, including right where those grinds are on the above image.  I've noticed the left side of the bolt stop area wasn't cleanly squared off on some of the previous lowers, but on this FDE and two others recently received they remedied that as well.  The front pin had the same issue, once more remedied by squaring off the material with a cutting pass.  All in all nothing major, but more evidence of a continued development process for the product.

There are still areas for improvement, namely that the back end of the fire control pocket seems to be an inconsistent location for their cutting operations.  Nothing functionally wrong there, but it sure is ugly and sticks out like a sore thumb on the FDE units.  I've griped a little about the front pin's detent spring hole being drilled too deep but again that's an extremely small problem and ends up making it easier to assemble.  Measured against an aluminum lower, the pistol grip block is well out of spec and tends to require a mallet to seat the grip fully.

Part 2 will cover some beating and maybe even involve video evidence!

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