Subsonic cycling problem

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

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MegaTon300
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Subsonic cycling problem

Post by MegaTon300 »

I just completed the assembly of my first blackout rifle build and I am having issues with it cycling on subsonic ammunition. I know they are meant to be fired suppressed with subsonics, but my understanding is that with an AAC barrel and gas block it should be capable of cycling subs without a can. My setup consists of an AAC barrel and gas block/tube, Bravo Company auto bcg, Spikes ST-T1 buffer and a Custom carbine center-less ground and polished operating spring made by JP Enterprises. Im thinking that my moving parts may be just slightly too heavy because my bolt is moving back far enough to eject the brass but not quite far enough back to catch the next round, but sometimes it cycles fine. Any advice or suggestions will be appreciated. Thanks
Turnpike
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Re: Subsonic cycling problem

Post by Turnpike »

I would find yourself a H2 weighted buffer. I have an AAC 9" upper. I used to run and Endine buffer when I ran the Noveske KX3. That was the only way my 7.5" would run. I tried the Eindine on mine without knowing how it would work. My round ejected. The Endine is similar in weight to an H3, but the hydraulics need to be compressed. The subs did not have the umph to do this. I put in a H buffer and it seemed to work fine, but I noticed the AAC manual and several others had better luck or got theirs to work with the H2.

Also remember ejection occurs much sooner on the 300 BLK than with the 556 sytems due to CASE length. I would find an H2 and have a go with that. Also what is the stength of your action spring. How does it compare to a standard M4. Before I went to my rifle class I am in now I replaced my older (3,000 round) action spring with a new Colt action spring and the H2 buffer just to eliminate any possible variables.


Mike
MegaTon300
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Re: Subsonic cycling problem

Post by MegaTon300 »

So a heavier buffer will travel better with the subsonics? It seems to me that the heavier H2 buffer would make for even worse cycling problems due to a lower pressure but of course Im no expert.
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Re: Subsonic cycling problem

Post by 300Blk »

The AAC 16 inch upper is designed to use a C buffer and a normal spring. Your spring is probably ok, but your buffer is too heavy.
MegaTon300
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Re: Subsonic cycling problem

Post by MegaTon300 »

Thanks a lot for the replies guys.

Rsilvers when you say C buffer you're talking about an H1 correct?
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Re: Subsonic cycling problem

Post by 300Blk »

No. An H1 is a heavy buffer. I mean a carbine buffer.
MegaTon300
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Re: Subsonic cycling problem

Post by MegaTon300 »

Thanks again Mr. Silvers.

My Spikes buffer weighs 3oz and all the other standard carbine buffers I have found weigh 3oz as well. Are there a certain type of carbine buffers yall use? Or should I try taking a weight out of the buffer?
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Re: Subsonic cycling problem

Post by 300Blk »

An ST-T1 is more than 3 oz.
MegaTon300
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Re: Subsonic cycling problem

Post by MegaTon300 »

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Re: Subsonic cycling problem

Post by 300Blk »

If it is in fact 3.0 oz, then it should be good for the AAC 16 inch upper.

But then they are being mistaken about it being a heavy buffer, as the lightest buffer Colt makes is the "C" buffer, and that is 3oz. An H buffer is 3.8 oz, an H2 is 4.6 oz, and an H3 is 5.4oz.
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