Cycling issues with subsonic load

Discussion about rifles in 300 AAC BLACKOUT (7.62x35mm), hosted by the creator of the cartridge.

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goalman50
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Cycling issues with subsonic load

Post by goalman50 »

Hello everyone. I'm new in this forum.

I have a 16" Aero Precision 300 blackout barrel, pistol gas length, gas block is not adjustable.

My supersonic loads performs flawlessly. I started loading subsonic load and that's where I hit a problem. My bolt won't lock back on the last round. The spent shell ejected fine and in the 4 o'clock position consistently. It just won't lock the bolt back.

I'm using Accurate 1680 powder, my bullet choice is the hornady 208 gr. ELD Match.

I try to look for some answer and where to start. Most info I found is suggesting to open up my gas port. According to Aero Precision, the size of my gas port is .0760". If I need to open it up, how much do i need to open it up?

Just looking for a direction to begin.

Thank you all.

Goalman50 8)
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bangbangping
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Re: Cycling issues with subsonic load

Post by bangbangping »

Opening the port should be a last resort.

What's your load? Do you know the velocity? 1680 is pretty forgiving, but of course a light load would affect cycling. Have you tried factory subs, particularly the Remington UMC? If those don't cycle, it's definitely a gun issue, not an ammo issue.

Buffer weight? If you have a heavy buffer, that can be an issue.

Properly lubed?

Make sure there are no leaks around the gas block and that it's installed properly.

Finally, how important is lock back, really? :mrgreen: Kidding, sort of. To me, for a rifle I'm just having fun with, it really doesn't matter much as long as the rifle cycles. For hunting, I'll neve burn through a whole mag. For self defense, I wouldn't be shooting subs in the first place. Just a thought.
goalman50
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Re: Cycling issues with subsonic load

Post by goalman50 »

I start the load @ 10.5 gr. up to 12.5 gr. with .5 increments. I only made 1 round of each load and chrono them. Unfortunately none of the rounds recorded any velocity and none of the rounds lock the bolt back, but ejected the case.

I haven't really used any factory subsonic ammo, because there are none available on our local Turner Outdoors (I live in the Peoples Republic of Kalifornia, just FYI).

I used the standard mil-spec buffer of 3 oz.

The gun was freshly cleaned when I fired those subsonic rounds.

I'm not quite sure about any leaks though. What test I can do to find out if there are any leaks on the gas block area?

Lock back is not really necessary, as long as its feeding the next round. That will be my next test. I load 5 rounds @10.5 gr. and hopefully to chrono it.

Thanks for the thoughts.
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dellet
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Re: Cycling issues with subsonic load

Post by dellet »

If it wouldn’t lock back with twelve grains, it’s not the load.

Leaks can often be found by looking for a carbon track. Places to look are coming out from under the gas block, where the tube goes into the gas block. Around the gas key were it mounts on the carrier, sometimes you can move a loose key.

A blockage in the gas tube is not uncommon on a new barrel. Look for copper traces in the carrier where the bolt is. It can come through the tube. Burrs on the gas port can shave the bullet.

If a suppressor is probably not in your future, opening up the port might make more sense, but it’s already larger than what I have seen work on a 16” pistol gas upper. I wouldn’t go over .080”.
300 Blackout, not just for sub-sonics.
goalman50
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Re: Cycling issues with subsonic load

Post by goalman50 »

I swab around gas tube meets the gas block and there is the carbon track you mention. In fact I tried to swab the barrel before the gas block and they are clean. it was dirty after the gas block. So if this is not normal then there is definitely a leak. I also check if the gas key is somehow loose, but they are not. Does replacing the gas tube with an adjustable one would solve my subsonic cycling issue?

Thanks again.
HK91
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Re: Cycling issues with subsonic load

Post by HK91 »

Try removing the gas block and do a couple measurements. From the back edge of the gas block to the center of the gas hole. Then on the barrel from the stop to the center of the gas port hole. Some gas blocks are set up to allow for a handguard retainer and if it is not there the gas block could be going back against the stop and be only partially over the gas port, ie not lined up.

Kelly
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