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Polymer AR Lowers

zurcher_czurcher_c Member Posts: 6

Please excuse my ignorance. I may be explaining this wrong.

I'm building a polymer AR in 9mm. I'm looking to take as much stress of the buffer tube thread area as possible. I seen the Armaspec RSR swap. Would that reduce the stress / recoil? I do know 9mm AR's operate by blowback and I'd need a heavier buffer if i was to run it normal, but to me it seems like that's adding more stress to the polymer application. Any help would be appreciated. Thank you.


  • yoshmysteryoshmyster Member Posts: 19,863 ✭✭✭

    Are you worried the plastic lower will break from the recoil where the buffer tube screws on? Unless you fall on the AR I don't think it'll break/snap at the buffer.

    As for the buffer choices I'd go with JP Enterprises Captured Buffer with the spring kit. With the spring kit you can fine tune.

    If you're really worried go aluminum and mill the fat off to lighten the load.

  • MobuckMobuck Member Posts: 12,878 ✭✭✭✭

    If you're not already too far into this plan, I'd suggest you just switch to a forged aluminum lower.

    The blowback 9mm creates a LOT of stress on the lower going both directions especially if the correct parts aren't used.

    The first impact occurs when the buffer hits the rear end of the tube(as expected). If the buffer and spring aren't working in unison, there's a hard impact at the end of the cycling stroke. Then the bolt slams forward (with the spring exerting force against the rear end of the tube) striking the rear of the barrel with considerable force which in turn produces stress in the opposite direction.

    The break is more likely to occur between the selector hole and the top of the lower as this is where the force and leverage is concentrated. If this is a 'pistol' lower, the problem is even more pronounced since theoretically, all of the recoil/cycling forces are directed to the grip rather than being taken by a buttstock.

    Regardless of aluminum or polymer, one of the things to be careful of is letting the bolt slam forward on an empty chamber. This pounds the snot out of a lot of parts.

  • NeoBlackdogNeoBlackdog Member Posts: 14,588 ✭✭✭✭
    edited September 22

    I believe there are companies out there that make 9mm uppers that require no buffer. I'll do some looking and see what I can find.

    Found this;

    Spendy and out of stock. This comes with a 5" barrel so it'd need to be a pistol or SBR.

  • pulsarncpulsarnc Member Posts: 5,477 ✭✭✭✭

    I help my buddy ( a full time gunsmith) from time to time Also manged a full line gun store for a few years selling tons of ar parts . Only problem i have been aware of is with the buffer retaining pin . Seen a few where it has worn and elongated to an egg shape and no longer does its job ,no way to fix that I am aware of . I guess one could try to jb weld it in place .

    cry Havoc and let slip  the dogs of war..... 
  • zurcher_czurcher_c Member Posts: 6

    This is all really great info. I do have the option to convert another AR i have with a forged aluminum lower. I just have this poly AR lower just sitting and I've been putting it off. The upper is a brigade manufacturing with a 5" barrel operating on blowback. Mainly my goal is to reduce the blowback as much as possible. Maybe it's a challenge I have yet to accept in my mind. = ) If I'm not mistaken, I need a heavy buffer around 5.4oz? I've seen hydraulic, the RSR from Armaspec and a few other options. What reduces the blowback the most is what I will buy.

  • MobuckMobuck Member Posts: 12,878 ✭✭✭✭

    The buffer should not be impacting the retainer during actual firing if things are correctly fitted regardless of whether this is a gas op or blowback. If this is a problem, the retainer can be removed w/o any functional issue other than being a PITA during breakdown.

    Some users have experienced breakage of the bolt catch as a result of the bolt traveling too far to the rear which caused the bolt to be moving faster than it should when caught by the bolt catch on a LRBHO equipped pistol caliber AR. This can be controlled by using the proper buffer spacer and choosing a different spring. I recently added a Sylvan folder to one of my AR9 PDW's and went through 3 or 4 spring/buffer combinations before finding a combo that allowed function w/o rearward overtravel.

    Buffer weight is a trial and error/guesswork proposition. I'd start with no less than 4.5-5 ounces. To accommodate the bolt extension of the Sylvan folder, I had to remove the plug from the 9mm bolt carrier which was partially compensated by the Sylvan extension although not completely. I had to change to a 7.2 ounce buffer to get back to the correct weight.

  • yoshmysteryoshmyster Member Posts: 19,863 ✭✭✭

    Maybe you might wanna get an adjustable gas block, too. Or go with a barrel that doesn't have a gas block hole so it'll be an AR bolt action.

    Final thought for the poly lower. Go for a .22LR build.

  • MobuckMobuck Member Posts: 12,878 ✭✭✭✭

    "Final thought for the poly lower. Go for a .22LR build."

    This is an option. Although I've not had any issues with polymer lowers, longevity IS a consideration. Back in 2010-2012, I had some of the early version 'PLUM CRAZY' polymer(ish) lowers for sale. These even had polymer FCG which I was very suspicious of. One buyer described his use as "a bullet hose" but no problems that I heard about. I still have one of those but with a metal FCG in place.

    I will say those polymer lowers come out really nice when finished using a 'jig' & router vs gouging around on a drill press AND it only takes about 15 minutes start to finish vs 45+ minutes for a forged 80% lower.

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