300 BLK bolt over brass

[Gunslinger55]

Bought a 10.5 upper pistol gas no name off board looked pretty good but as I suspected has issues. I know nothing about 300 blk so I thought I’d experiment with this cheap. Use a sbr carbine lower,frontier supersonic ammo. 3-4 round fails to feed bolt over brass. Tried different mags and heavy buffer same issue. Seems like maybe over gassed? Locks back on last round though. Any suggestions before moving it along and getting a BCM? Thanks

 

[YNOTAZ]

"Bolt over brass" Is bolt retracting all the way? 1 round in Mag does bolt lock open when fired? Does brass eject between 3-4? Doe brass eject @ 4+? or 1-2?

Maybe lighter rather than heavier on the spring/buffer?

 

[Gunslinger55]

Ejection fine. Failure to feed on 3-4. Load one round bolt locks back. I’m thinking from what I’ve researched that bolt traveling to fast to pick up rounds lower down in mag? Apparently a common problem but just trying to diagnose. I tried heavier buffer and that did not work though so looking for advice. Different ammo ? Tried different mags. Seems maybe over gassed but not sure

 

[Kingjoey]

Try a heavier buffer

 

[Gunslinger55]

Used an H3. Probably overgassed I’m guessing

 

[Gunslinger55]

Ok problem solved. I swapped in my enhanced LMT bolt and carrier with H3 buffer. Gun ran fine on supersonic 125(only ammo I tried). The LMT delays cycling so it is obviously overgassed. A adjustable gas block would work too I’m guessing.surprised with pistol system must have a big hole in barrel

 

[Viperdisorder]

Pistol length barrels are real touchy for gas to buffer-spring tuning as you guys know, been using armaspec stealth spring kits, they are not cheap, but in addition to being quiet, they have two stage spring system , that delays and slows the unlock and return

 

[Harrier]

If it is locking open then the bolt should be going back far enough. Are the two carriers the same tail cut/weight?
sounds like it is short stroking (but if so, the bolt hold open is baffling) - If it's short barrel I would lean more toward under-gassed and I would think you would want a larger port - like indicated, maybe its a timing issue.

Try the old BCG and run it really wet-lube it good if you didn't before.
Sometimes the BCG needs to be broken in to the receiver-too much friction until the parts get worn together.

 

[Kingjoey]

Another issue with 300BO is that the pressures and burn rates for supersonic and subsonic ammo are vastly different. Its like two different cartridges so the port sizes can be a bit different too. If I had a 300BO I would set it up specifically to shoot one or the other rather than trying to get it to work with both.

 

[Gunslinger55]

Yep i figured that out and fixed the overgas with supersonic. Subsonic runs standard boltcarrier and buffer supersonic needs a delayed enhanced bolt and a H3 buffer then runs fine. or just switch out the buffer and run the delayed carrier. I use LMT enhanced. Just needed to slow it down a bit from a pistol gas systen 10.5" BBl

 

[Gunslinger55]

If it is locking open then the bolt should be going back far enough. Are the two carriers the same tail cut/weight?
sounds like it is short stroking (but if so, the bolt hold open is baffling) - If it's short barrel I would lean more toward under-gassed and I would think you would want a larger port - like indicated, maybe its a timing issue.

Try the old BCG and run it really wet-lube it good if you didn't before.
Sometimes the BCG needs to be broken in to the receiver-too much friction until the parts get worn together.

Its not short stroking its going so fast the round does not have time to pop up and get caught by bolt. Happened from going 223 to 556 on one of my shorties as well put a heavier spring and and no issue. Its amazing how much more pressure a 556 is than 223. Kinds like sub and super 300blk

 

[shooter444]

:text-+1:

Another issue with 300BO is that the pressures and burn rates for supersonic and subsonic ammo are vastly different. Its like two different cartridges so the port sizes can be a bit different too. If I had a 300BO I would set it up specifically to shoot one or the other rather than trying to get it to work with both.

I fiddled with this problem, when I first got into 300BLK, and wound up compromising with heavy sub bullets at around 1,400 FPS. I only want it primarily for less than 50 yard work, so, it suits me fine, like this.

I read about many nightmares of guys trying to get their 300BLKs to handle both sub and super ammo. I think the first move would be an adjustable gas block, with a pistol length set up.