Polymer 80% AR lowers-from my perspective

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Mobuck

Hawkeye
Joined
Dec 25, 2007
Messages
7,158
Location
missouri
For want of better place I'm starting this thread here in the 'smithing' area.
If anyone is thinking about entering the POLYMER 80% lower market, I'll offer some observations.:
1-While you CAN do the gouging on a drill press, it's far better if you invest in an X/Y axis vise for the press. It's slow and requires patience and care but it CAN be done successfully. That said, quite often there will be Dremel handwork involved in producing a working part-again, slow and careful.
2-It's a fairly easy 15 minute job to cut one using an actual router 'jig' and produces a useable product only requiring some clean-up of burring around holes.
3-There are several versions some of which use a separate nut for the pistol grip and some that use a brass insert cast into the polymer. Also some use a captive pin/set screw to hold the bolt catch in place while others use the common roll pin(be darned careful when driving that pin in place). Most/all are designed with a 'closed' trigger guard. One reason for this is strength-the weakest part of the AR lower is the area where the trigger pin and trigger hole cut is located. The 'closed' trigger guard adds strength and rigidity to this somewhat weak area.
4-Quality of the 'cast' is variable (and these are only my observations). JMT seems to be a good product-I found no voids or porosities in the internal cutout areas. Polymer 80 RL556v3 comes in as #2 with only limited porosities and virtually no voids. The Polymer 80 had several issues with porosities, voids, and internal imperfections. These are actual observations of several of each version.
5-Regardless of brand, I found that there's a considerable variation in tolerances and not just any combination of polymer lower will fit with any/all uppers. It's like a big game of mix & match with pairing up combinations that fit. So don't expect to just take home upper and lower and have them fit properly. Just a SWAG that 10 lowers and 5 uppers will result in 5 sets that fit and that might be a bit high. I had to do a LOT of hand fitting on some.
 

RSIno1

Hunter
Joined
Sep 17, 2013
Messages
2,238
Location
Southern California
I have never liked the idea of a poly lower for an AR. There was a pretty big build thread on CalGuns about 80% builds and most recommended the poly be limited to a 22LR upper.
I machined out my first lower from a raw forging in which only the mag well had been broached. I had a real mill in the garage at the time and was off work due to an injury so the time consuming task was more like boredom therapy.
After that I switched to a router and haven't looked back. A drill press spindle speed and accuracy leave a lot to be desired. 80% Arms now has a Gen3 router jig. I've got their Gen2 one and have done 6 AR10 & AR15 platforms on it using both forgings and their CNC lowers. I have more to finish including a 9mm lower but I'll wait until we move to a free state since CA now requires you to serialize and register 80% builds. I have a pistol 5.56 upper but I'm waiting to see how the whole fed pistol/SBR thing sorts out.
I've had no mix/match problems with my lowers and uppers. Most of the uppers I've used were cheapy AR Stoner ones from Midway. I've hand built a couple .308 AR10 and a heavy barrel AR15 target upper though.
https://www.80percentarms.com/products/easy-jig-gen-3-multi-platform-ar-15-ar-9-and-308-80-lower-jig/
 

Mobuck

Hawkeye
Joined
Dec 25, 2007
Messages
7,158
Location
missouri
I've been very satisfied with the 80% Arms Gen 3 jig after a couple of 'start up" issues. The router bit included with the jig kit literally exploded the first time the router came up to speed. Fortunately, I wasn't hit by the shrapnel which left several marks on a nearby metal cabinet. The company sent me a replacement which was a good show of customer service.
FWIW, I've seen poly lowers used hard with little/no care that gave good service.
 

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