AR .300 BO Subsonic - 220g function question

[1-12 INF (M)]

Using Berry's plated 220's and 9.5 to 10gn of H-110, these cartridges seem to work fine in published data. For me, they don't function reliably - bolt fails to come back all the way and cycle.

I don't have a suppressor on my test gun, the tests were run with one. Does the can make a difference in rifle operation?

Shooting a 16" upper.

Thanks

 

[gotboostvr]

It's hard to day for sure if a can would "fix" your problem.

It some cases it might, in other cases it might not.

If you want to run that exact load maybe try an adjustable gas block.

 

[Varminterror]

Does the can make a difference in rifle operation?

Yes. Can = more cycle energy.

Whether the increase is enough in your specific rifle with that specific ammo to cycle, rather than some otherwise defect in your action notwithstanding. But suppressors put a lot more energy into the gas system.

 

[Varminterror]

If you want to run that exact load maybe try an adjustable gas block.

This relies upon a sufficient port size to allow cycling. AGB’s are all “less,” no “more.” If it won’t cycle as is, it probably won’t cycle with an AGB. Reaming the gas port to a larger dia then throttling back with an AGB is a sensible solution.

 

[1-12 INF (M)]

Thanks - I suspected the can would help in this case. I don't have one, so this is as good an excuse as any to get one...

 

[gotboostvr]

This relies upon a sufficient port size to allow cycling. AGB’s are all “less,” no “more.” If it won’t cycle as is, it probably won’t cycle with an AGB. Reaming the gas port to a larger dia then throttling back with an AGB is a sensible solution.

That's very true.

I had a much longer response written up and parsed it way down as it started to ramble. Accidentally left that bit in.

 

[Havok7416]

I use the exact same bullet but I don't use H110, although I did experiment with it. If I did use H110 in Blackout it wouldn't be for subsonic rounds. By the time you have put enough powder in the case to cycle the gun you are looking at a supersonic bullet at the muzzle.

ETA: The can doesn't matter

 

[1-12 INF (M)]

Havok - I didn't have success with IMR-4227 either. Any other ideas?

 

[mcb]

What length gas system on that 16-inch barrel? Carbine or longer is going to give you issues with subs. In my experience 300 BO is most reliable over the broadest range of ammunition and suppressor use with a pistol or shorter length gas system.

As other have indicated suppressors do increase bolt velocity of an AR-15. The suppressor holds pressure in the barrel longer and this increased pressure in the barrel allows, once the regular gas system unlocks the bolt, the spent case to act like a gas piston itself increasing bolt velocity.

I have not had good luck with H110 in subsonic loads, love it for supersonic loads. For that particular Berry's bullet I use IMR 4227 with good success in my 9-inch and 5-inch guns. You might try Accurate 1680 if 4227 is not working, it produces even more gas.

Ultimately you might have to increase you gas port size. I know I had to increase the gas port on my 5-inch barrel quiet a bit to get it to run reliably, but it started out with a silly small gas port.

 

[Havok7416]

4227 rocks for Blackout loads. It's actually all I use right now. If you have a carbine length gas system it usually won't cycle though.

Other powders that people use for Blackout include 1680 and CFE-BLK but I don't have a use for either of those in any other caliber and I like to keep things simple.

ETA: mcb beat me to most of this by a few moments (and in more detail!). I will add to his comments that while the can does increase pressure inside the gun, IME it was not enough to push the BCG all the way to the rear in the 3-4 Blackout rifles with carbine gas systems that I tried.

 

[mcb]

The bullet on the right is the OP bullet in question and works well with 4227 for me. I could never get the bullet on the left (Maker REX 220gr) to work with 4227 but found 1680 to work very well with that bullet.

 

[Havok7416]

The bullet on the right is the OP bullet in question and works well with 4227 for me. I could never get the bullet on the left (Maker REX 220gr) to work with 4227 but found 1680 to work very well with that bullet.

I'm guessing the REX just doesn't leave enough space for powder.

 

[mcb]

I'm guessing the REX just doesn't leave enough space for powder.

I could get the powder in there but due to the all copper construction of the REX takes up more case volume, causing increased velocity and by the time I reduced the load back down to subsonic I did not have enough gas to cycle. I am running 10.1gr of 4227 under the Berry's and it only took 9.4gr to push the REX to the same velocity. 1680 works well with the REX but not quite as quiet.

 

[Varminterror]

That's very true.

I had a much longer response written up and parsed it way down as it started to ramble. Accidentally left that bit in.

No worries and no conflict. I was just adding to your foundation. The statement is true, and especially so in common practice. Most gas ports are oversized to ensure reliable function, so tuning gas usually only involves tuning gas DOWN. Exceptionally rare that a guy really needs MORE gas than available - and when they do, it typically means binding, blockage, or leaking somewhere, not that the port was undersized.