7" 300BO Palmetto State Complete Upper and lower kit with 'Full Auto' BCG, standard carbine buffer (2.8oz), and spring.
208 Grain Hornady Black factory loads.
Amend 2 5.56 Magazine and Magpul 300BO Magazine
Unsuppressed - stove pipes ejected brass every time.
Suppressed - will eject brass with no deflector strikes at about 3:00, but will not cycle next round out of the magazine into the chamber. It will either go fully into battery on an empty chamber or will catch the round halfway into battery between the feed ramp and magazine.
Suppressed - Removed 1 weight from buffer - no change
Suppressed - Removed 2 weights from buffer - no change
Suppressed - Removed 3 weights from buffer (no weight in buffer) - will run semi-auto for about 3 rounds and will either go into battery on an empty chamber or catch the round halfway. Also the bolt will not lock open after the last round, but will lock on empty mag manually.

I have an Odin Works Adjustable (2.1 - 4.2oz) AR15 Lite Buffer on the way for Monday delivery.

What says the hive:
Buffer still needs to be lightened up?
Needs empty buffer with lighter or shorter buffer spring?
Heavier (H1/H2) Buffer?
Adjustable Gas Block?

I ran in to the same issue with a brand new upper i assembled. The difference being my can is an OSS (do NOT recommend for 300blk, it's loud).

Barrel is a 7 inch Q barrel with an m16 carrier.
With the gasblock wide open, it would not cycle 220 gr subs.
I replaced the carbine weights with aluminum and a lightweight buffer spring. It cycles now but the recoil is harsh for 300blk.

In the end, the upper components needed to "wear in" to each other.

I lubed the crap out of the bcg and ran a magazine of supers through it: it cycled great, ejected the brass at 3 o'clock, and locked open on the last round.

I'm now able to utilize the adjustable gas block :)

If it were me, I would be looking into it being under-gassed first. Try running supersonic ammo and see if you notice any change. As for the weak extraction, if you have another bolt you could try, that is an easy thing to rule out.

I suspect under gassed with factory loads (subsonic out of 16"bbl?).

I would run a heavier bullet in the 900 fps range. Maybe 220gr - 245gr.

I run 245 gr coated cast bullets using 7.7 gr of 4227 which are about 770 fps. (plinking round).

You just don't have the dwell time the longer barrels have so a bigger slower bullet helps. At least thats how I see it.

8" barrel
T3 buffer
Titane Carrier
Full open adjustable gas block
7.7 4227 w/ 245 gr cast.

ETA: I like the lightweight carrier with the heavy buffer.
I get less overall reciprocating mass but still have a heavy buffer to help get those cast bullets all the way home.

Feed ramp opened up to avoid scarring bullet during feeding

Runs suppressed and unsuppressed.


This is what I used to model my feed ramp adjustment.

7" bbl as stated in OP. 208 gr is all I have and could find. I have looked around everywhere locally and online for 220 gr. I don't reload anything so that would take a lot of start up capitol.

Extraction is only weak unsuppressed. Suppressed it ejects +3:00- every time (loading the next round is the issue). Once I take full possession of my AAC Suppressor this will be a 100% suppressed weapon.

I was reading this AR Build Junkie Article:

AR-15 Gas System Issues – Over-Gassed Misdiagnosed as Under-Gassed
I had a guy I helped recently on one of the Facebook groups that had a gun that functioned fine until he put a suppressor on it.  It would do a “bolt over base” malfunction. The projectile is in the barrel extension, and the back end of the casing is still half way in the magazine.  The bolt lugs were jamming into the side of the casing.

If you looked at the comments of people trying to help him, 95 percent of the people would say that gun is under-gassed because it couldn’t come back far enough to strip that round out.  But what was actually happening is the bolt was moving so fast the magazine could not keep up.  

The bolt went back really fast and came forward really fast, and before that round could completely present itself, it was hitting the side or just barely missing the back of the casing and jamming it up.

The fix there was to tame the system down by putting a Black River Tactical under sized gas tube in there…so it had a restricted port, and then the problems went away.

The bold statement above is exactly what mine was doing until I started taking weights out of the buffer. Next range trip I am going to put all of the weights in the Odin works adjustable buffer and take the Medium and Heavy Springs from a Franklin Armory BFSIII kit that I have unused from my 9mm build. Start with the 'Heavy" Spring and all weights in the Odin Works Buffer then go down from there until it runs.

FWIW: My CAR Buffer originally weighed 2.8oz.
ETA: The unused springs from the Franklin Armory BFSIII are Rifle length so I'm going to use the original Carbine length spring with the Odin Works buffer.

How does it cycle with supers?  If fine unsuppressed, then your gas port is too small for those subsonics.

I've never shot it with Supers & I don't have any.

Ended up finding some supersonic rounds at a semi-local pawn shop. Next range trip I will try them out with the original carbine buffer.

In other news, I built a 10.5" 5.56 upper, after assembling it and seeing how installing a non pinned gas block leaves a lot of room for error I decided to pull the rail off of the 300 and look at its gas block. The gas block set screws were looser then I thought that they would/should be and it was neither completely square to the center line of the barrel nor was it seated against the barrel flange. I loosened it up and found powder residue to one side of the opening in the barrel. I reinstalled the gas block making sure that it was square with the barrel and fully seated on the barrel flange. We will see how she does next range trip.

The right combination:

Drilled the gas port out to 7/64 (0.109)
Sprinco 'YELLOW' reduced power Spring (20% less)
Carbine Buffer with all weights removed
Locked the bolt back on empty magazine and cycled semi-automatic Suppressed with both Hornady 208Gr & Remington 220Gr.

In my fairly limited AR building experience - about a dozen various ARs in a mix of 5.56, 300BLK, and 6.5 Grendel - I have yet to find a gas port located such that the gas block needed to be seated against the shoulder.  I use an SLR barrel dimpling jig to locate a dimple at 180° from the gas port, and invariably the gas block sits right at 0.25" from the shoulder - the thickness of a standard front handguard cap.  I realize that some barrel and gas block manufacturers adjust their dimensions to eliminate the gap, but I haven't come across one yet.  Also, many gas block manufacturers oversize the hole in the gas block to allow for slop in the alignment.

I think that .25" should be .025". Yes, I compensated for the handguard end cap. You are correct that most gas blocks are drilled at .125", but there was definitely an alignment issue at first. After enlarging the gas port in the barrel to .109" I used a pressed paper dowel to line up the gas port in the barrel and the gas block then knocked the dowel out with a barrel rod. It would have been nice to have had access to a barrel dimpling jig though. I may look into getting one just to dimple the barrels on all of mine.

PSA 7.5" 1:7 .300 BO had the same issue with 190gr subs.  Wouldn't cycle or lock the bolt back...
You've probably got a 0.097" gas port.

Drilled out gas port to .120" but still wouldn't lock back or pick up next round.
Exchanged buffer spring for lighter variant from Brownells (100-011-674) and a clamp on adjustable gap block .  Now the bolt reliably locks back.

Still having some feeding issues brought about by the lighter spring and also an issue with the feed ramps that don't like hollowpoint ammo.  Going to need to do some dremeling.

Take the weights out of your carbine buffer too.

Why the adjustable gas block? You needed more gas not less gas. Full open on an adjustable gas block is the same as a standard gas block.

Did you completely remove the bottom locking lug?

Yes understand that, but the original gas block was not a good fit (leaked), so I replaced it with a clamp on adjustable block.  Ideally I'd like to be able to shoot supers as well, so the thought of an opened up gas port, lightened spring and lightweight buffer made me think I'd need a way to manage the extra energy from the hotter load.

That makes sense now.

I just bought the KAK Industries lightweight buffer set, going to try those out on the next range trip with the Sprinco Yellow Buffer spring (reduced power). I'd like to have a little bit of weight in the buffer. Even with the empty buffer there's less recoil than both of my suppressed 10.5" 5.56s.