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Would that make the groups tighter?No 300BLK experience here, but:
Sounds like you need a powder that produces more gas. I would try CFE BLK, 1680 or 4198.
Or
Go with a lighter buffer spring/buffer setup to make LiL Gun work for you.
I changed the coal on this last batch of test rounds and it helped a little but still not quite like I like I think I’m going to get some 1680 and cfe blk and see if that makes a differenceThe group size may or may not shrink by changing powder. You just have to experiment when picking a powder. The crimp, and COAL should also be varied in the search for accuraccy.
The bolt is more likely to lock back after the last shot with more gas or a lighter spring/ buffer combo.
Another item to frustrate yourself with is the primer flash hole and uniformity. Some guys enlarge the hole for subsonic loads. One of my range friends told me that enlarging the flash hole helped him. Check out the flash holes in the factory subsonic.
I have found that on my guns, one one the biggest things that affects accuracy is consistant brass—same brand, same neck tension, same primer hole, same capacity.
8" barrel with carbine gas? You need a different barrel for those subs.
And small base dies to run all your brass before loading.