New build woes...

AR/M4 Beyond 5.56/.223
1

Just finished a CMMG based 300blk 8.2" barreled SBR build.

receivers, barrel, bolt/carrier, barrel nut: CMMG
LPK: spikes
RE/stock, gasblock: BCM
Trigger : ALG

my issue is that i’m struggling with the bolt not locking back using subs (suppressed/unsuppressed) after the last round fired…but only if there is more then 1 round in the magazine…if i load just one round, it will lock back with a carbine, H, H2 buffers…if i load more than one round it won’t lock back. It’s weird and i’m confused.

Thanks in advance.

2

What magazines have you tried?

3

Lancer & Magpul

4

Does it work with supersonic loads and does the lower function with 5.56 uppers?

5

yes it works with supers, and i’ve not tested the lower with any other upper.

6

I’d try more subsonic loads before trying to open up the gas port. Good luck.

7

Are these factory sub loads? Which buffer are you using?

Had similar issues with mine (also 8’5" barrel) but my issue turned out to be an undersized gas port.

8

Gorilla Subs and Gemtech Subs…tried carbine, H, H2…all same issues…put one round in the mag and it would lock back, put more than one in the mag and it wont.

9

Uh…perhaps I’m not understanding what you’re saying, but if there is more than one round in the magazine, it’s not supposed to lock back.

10

Load 1, insert mag, fire= lock back
Load 2, insert mag, fire x2=no lock back

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11

A stronger buffer spring? I was thinking that it wasn’t returning with enough force to strip the next round from the mag, but that doesn’t seem to be your problem. That is kind of baffling. I would say sell it and get a .300AAC if you’re going to be shooting a lot of subsonic, that’ll fix your problem.

12

It is a 300blk? Or you mean get an aac upper?

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13

Never used gorilla or Gemtech loads, try another manufacturer sub rounds.