I took out an SBR to the range today with two uppers.  The first, a 10.5" with a new Battlecomp ran perfectly.  I switched to a new 11.5" upper which I shot once before without a suppressor and it ran fine for the 120 or so rounds I ran through it.  Today I took the 11.5 out with a Gemtech M4-02 attached and ran into FTF's about every other or every third round.  The bolt was riding over the top round coming out of the magazine.  I was shooting LC 193. Unfortunately that was the only ammo I had with me.   I tried three different mags, all have never missed a beat. (20rd Pmag, 30rd Pmag and a 30rd USGI)  The lower has an H buffer.

Any ideas where the problem is coming from?

Sure was nice shooting an SBR without hearing pro though...

FB

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If I understand what you are saying correctly, sounds like the bcg is flying forward past the new round before the round can be pushed up from the mag tension.

That's how I read it. Increased bolt speed due to the extra back pressure from the suppressor. This is the reason so many companies have started to use adjustable gas blocks for suppressor use. Try a heavier buffer to slow the bolt down.
 

Since I use this lower with other short un-suppressed uppers would it make more sense to use an adjustable gas block on this upper?  Got one you'd recommend?

Thanks for the help guys.

FB

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+1 on a heavier buffer. This is probably the best thing to try first.

+1 ..and cheapest to try first    and if it doesn't work, then that is spare parts for you to have on hand

Do you think going to an H2 or H3 would be the way to go?  It's an H now.

FB

I ordered a Spikes ST-T2 should be here in a couple of days.  I'll post a range report.

Thanks for the help guys.

FB

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9mm buffers add a lot of bolt bounce in 5.56 uppers and belong in 9mm uppers
the ST T2 does nothing better than an H buffer with more bounce.

do a search for the videos/threads 87GN did on various buffers and bolt bounce

very informative

I just tried a dozen searches with different parameters and found nothing.  How long ago were the videos posted? Which forum?

FB

Interesting viewpoint.

This may not have any relevance for SBRs, since my experience is primarily with full-auto ... but I used both Colt 9mm hydraulic and Colt 9mm two-piece mechanical buffers as my primary buffers in M16s for many, many years. Then again, all my uppers were factory Colt, so that may have something to do with it ... but all ran 100% with those buffers in FA and semi: 9mm uppers in 7" and 10.5", integrally suppressed LRM M169 upper, 5.56 in 11.5, 14.5, 16 and 20" uppers.

At the moment I'm running an H2 primarily for all functions (and all are reliable with it) ... not that I intentionally switched, just that I'd thought I'd try one on a longterm basis. I'm still waiting to find a combo in which it does not run 100%.

Maybe I'm lucky. Then again, I'm still trying to find the right buffer to get my LMG upper to run 100% with a collapsible buffer ... and as far as .22RF, let's not go there, nothing works. For me.

I'm a great believer that individual tolerances, and stacked tolerances, mean that there is no absolute ... for buffers, for barrels, for gas-port size, for ammo, etc.

Again, YMMV.