Buffer / Spring Recommendations for 9mm AR

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  • ranger391xt

    Plinker
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    Feb 27, 2019
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    West Lafayette
    I purchased a 9mm AR upper this past weekend, and after getting home realized I should have also picked up a buffer and spring as well. Looking around online, I see that buffers for 9mm can range anywhere from 4 oz to 8 oz in weight. I assume that the proper weight for the buffer depends on things like the weight of the bolt and the buffer spring itself. I am planning to use this on an MP 15 Sport lower, so I know I need a carbine length spring if I change that out.

    Any recommendations/guidance on what weight buffer and spring to use? Would a carbine length spring for an AR-10 be OK, or do I need something heavier/lighter?
     

    ranger391xt

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    Feb 27, 2019
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    West Lafayette
    Thank you both for the feedback.

    Based on the info in the blog which indicates the buffer and BCG should have a combined weight of 22-23 oz., and the BCG in my upper weighing 14.6 oz., I have ordered an 8 oz AR Stoner buffer from Midway.

    The blog indicated a heavier spring isn't necessary, but I did go ahead and order a flat, Teflon PVC coated ar-15/LR-308 spring to reduce the noise in the buffer tube.

    Looks like they won't get here until next week, so have to wait a few more days to try this out.
     

    mike4

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    Mar 23, 2010
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    I just recently posted into another thread about the Colt pattern 9mm AR recoil assembly setup. My Colt 9mm buffer, purchased maybe 8 years ago, is like that shown in the article "The Original 9mm AR Deadblow Buffer" at marvin02's link. I'm unhappy to read in that article Colt has cheaped out on the current production buffers. I would have preferred to order a spare had I know that was coming to potentially setup a 2nd lower without swapping the buffer between them every time, although that 9mm upper build was already costing a small fortune in Colt parts + Hahn magwell at the time. (A lot less 9mm AR options back then, the Glock mag setups still had issues and I wanted to be able to swap it on to a 5.56mm AR SBR lower.) I suppose H3 buffers will have to due as a spare.

    Your combined weight comments had me interested to check the weights of those Colt components including BCG purchased in the same time frame. I just pulled them out and weighed them. Buffer is the correct 5.5 oz, 9mm Colt BCG is 15.9 oz, so 21.4 oz combined.

    Standard recoil spring should be fine. Bolt/buffer weight helpfully increases inertia where you want to delay the direct blowback motion of the BCG. The spring has less influence at that point in the cycle as it is extended, and the spring force increases as it is compressed, so this extra force of a stronger spring is applied at an unhelpful time later in the cycle where it could result in short stroking (or a system much closer to failure with less powerful ammo, getting dirty, applied lube getting sluggish in the cold, etc.)

    Possible the heavier spring is in a range where it still makes little difference, but with the Colt 9mm masses developed around the standard carbine spring I just would not push in the the direction of a stronger spring (and it's very simple to stock the same spare springs for the 9mm and 5.56mm M4 lowers).
     
    Last edited:

    marvin02

    Don't Panic
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    Jun 20, 2019
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    In case someone is looking for an AR9 buffer:

     

    ranger391xt

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    Feb 27, 2019
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    West Lafayette
    Buffer (8 oz) and new spring (AR-15/LR-308/AR-9) arrived yesterday. Put them in and ran about 50 rounds through it. I think there were maybe two or three rounds that did not eject properly. Other than that it seems to eat everything I fed it - factory ammo and reloads, Round nose, hollow points, TC's.
     
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