My 300 Blackout will not cycle subsonic

[Osage48]

My 300 BO will not cycle using subsonic ammo with a can. It cycles fine with supersonic ammo. I have tried a lighter spring and heavy lubrication with no success. Is there anything else to do other than open up the gas port?

Anyone know of someone that can open up the port in the Ada area? Suggestions?

 

[Sanford]

What barrel and gas tube length? "Not cycle" can mean several things - does it extract and eject the fired round but not feed the next round, something else? If it's ejecting okay where does the spent brass land? If it's not feeding the next round you might try a lighter weight buffer or a different magazine or different type of magazine with a different spring rate. Also guessing that you've already checked that your gas block is properly lined up with the port; opening up the port would be my last resort unless it's one of the barrels known for having small ports to begin with (have to search around the net, don't remember which they were but there was some discussion of them a while back).

 

[gmar]

Are you running a can with your subsonic and are the subs factory or handloads?

 

[Osage48]

It will eject the spent brass, but not pickup a new round. I have tried a lighter weight buffer as well. Not sure how to make sure the gas port is properly aligned. It works well with supersonic ammo. I am using reloaded ammo (220 gr jacketed) and have shot thru a chronograph at about 1050 fps. The barrel is 16" with a mid length. I have used multiple mags with no success. The rounds cycle smoothly by hand so I don't think that is the issue. I am using a can.

 

[gmar]

What's the load data? If your rifle is like mine, there's a fine line on getting it to work. For example, shooting 208g Amax with 10.7g of Hodgdon 4198, it functions great and keeps the bolt open on the last round. With. 10.5g it functions but doesn't keep the bolt open. Both average 1000-1050 fps. I have a CMMG 8.5" barrel.

 

[aestus]

Did you say 16" barrel with a midlength gas system? Like the same mid-length gas that's on a 5.56/223 rifle?

If so, that's your problem. 300 blackout uses very fast pistol powders and the pressure spikes optimum for cycling happens early. 300 blackout optimized with pistol length gas systems. For 16" barrels, you can get away with a carbine length gas if you know your loads, but it is still often recommended that a pistol length gas is used even for a 16" barrel. Using a longer gas system will result in not enough pressure going through the system to reliably cycle the rifle as the pressures will already have come down by the time gas is being pushed to your bcg.

 

[Sanford]

With a 16" barrel and mid length gas system it 'should' be able to cycle using subsonic ammo even without a can. It's usually the shorties - under 10" - that need the can to cycle with subs.

Have you tried generously lubricating the buffer spring? I once had a problem similar to that you describe and a bit of white lithium grease rubbed up and down the buffer spring solved it.

Unless it's a pinned gas block you should be able to check alignment by loosening it and sliding it forward to check the soot mark around the barrel port (most gas blocks are drilled enough larger than barrel ports that the leave an easily seen ring). If not right, adjust as appropriate when you put it back. Oh - and might as well check the gas port size while it's open, too. There's a pretty good discussion of port sizes here: http://micromoa.com/300-blk-port-size-testing/

Does the bolt lock back on an empty magazine shooting the subs? Have you tried the "plunk test" - BCG removed, muzzle pointed down, manually dropping a round into the chamber should go "plunk" ... if it doesn't easily go all the way in you might need to compare with some factory ammo to see if maybe you have a sizing issue. In fact, getting some factory subs for comparison might not be a bad test in any case.

Can't really think of much else to post at the moment, other than "good luck!"

 

[dennishoddy]

Did you say 16" barrel with a midlength gas system? Like the same mid-length gas that's on a 5.56/223 rifle?

If so, that's your problem. 300 blackout uses very fast pistol powders and the pressure spikes optimum for cycling happens early. 300 blackout optimized with pistol length gas systems. For 16" barrels, you can get away with a carbine length gas if you know your loads, but it is still often recommended that a pistol length gas is used even for a 16" barrel. Using a longer gas system will result in not enough pressure going through the system to reliably cycle the rifle as the pressures will already have come down by the time gas is being pushed to your bcg.

With a 16" barrel and mid length gas system it 'should' be able to cycle using subsonic ammo even without a can. It's usually the shorties - under 10" - that need the can to cycle with subs.

Have you tried generously lubricating the buffer spring? I once had a problem similar to that you describe and a bit of white lithium grease rubbed up and down the buffer spring solved it.

Unless it's a pinned gas block you should be able to check alignment by loosening it and sliding it forward to check the soot mark around the barrel port (most gas blocks are drilled enough larger than barrel ports that the leave an easily seen ring). If not right, adjust as appropriate when you put it back. Oh - and might as well check the gas port size while it's open, too. There's a pretty good discussion of port sizes here: http://micromoa.com/300-blk-port-size-testing/

Does the bolt lock back on an empty magazine shooting the subs? Have you tried the "plunk test" - BCG removed, muzzle pointed down, manually dropping a round into the chamber should go "plunk" ... if it doesn't easily go all the way in you might need to compare with some factory ammo to see if maybe you have a sizing issue. In fact, getting some factory subs for comparison might not be a bad test in any case.

Can't really think of much else to post at the moment, other than "good luck!"

Agree to try factory subs. I think aestus is on the right track. Pressure rules in this scenario, and it sounds like the gun is short cycling by a tad. Short barreled .22 mags and .17 HMR's have the same pressure spike issues that he is talking about.

 

[Osage48]

I appreciate all the advice. The reloaded rounds do plunk into the chamber and fall out if you point the rifle up. The bolt will not lock back with an empty mag. The gas tube is pinned. I am using Lil'gun powder. I will try some other loads and see if I can keep sub and still cycle.

 

[Sanford]

I am using Lil'gun powder. I will try some other loads and see if I can keep sub and still cycle.

AA1680 seems to be the "go to" powder for subsonic loads if you can find it. Several others with results listed here ... hope it helps!

http://www.300blktalk.com/forum/viewtopic.php?t=86656