Upper = stock CMMG complete 8.5" pistol gas upper.
Lower = stock Colt carbine
Buffer spring = stock Colt carbine
Ammo = factory Remington 220 gr. subsonic
Magazine = stock Colt 20 and 30 rd. aluminum
Suppressor = Yes

Buffers I've tried:

stock carbine weight (3.0 oz), results: Failure to lock back on last round and many failures to feed new round into chamber, i.e., will fire and eject fine, but either the timing is off and the next round doesn't have enough time to move up and get picked up by the bolt, or the bolt is not going back far enough to pick up a new round (bolt closes on an empty chamber).

H (3.8oz), results: Failure to lock back on last round, but cycled fine.

H2 (4.6oz), results: Failure to lock back on last round, but cycled fine.

Before I spend more money on an H3 buffer am I going in the right direction? Maybe a different recoil spring? Thanks for any help.

Have you tried a CAR buffer yet?

edit- NVM, can't read this morning

If your referring to the standard 3.0 oz carbine buffer, yes, that was the one that had the failures to feed in addition to the failure to lock back on the last round.

Im just starting out with the 300BLK but that sounds like you might also have a gas port issue. Is the CMMG adjustable?

what he said ^

pull off the GB and measure the port.

I don't recall the "standard" (if you will) port measurements, but that'd be a good place to start.

As already mentioned, see if supersonic ammo will make the bolt stay back after the last round.  If so, then see this link for some 300BLK port size testing: http://micromoa.com/?page_id=496

It's fixed! I kept the H2 buffer in it just because that was what is in it now, I really poured lubricant to the bolt carrier group, and used a newer 30rd. GI alum. magazine with a green follower. Cycles and locks back on the last round now. Thanks for all your replies.

Glad to hear it. I saw your post over on 300blktlk and noticed those guys going the mag route instead of the port. Much cheaper!