Subsonic Unsuppressed 300blk from 8.3" Barrel

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fancygunz
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Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by fancygunz » Thu Aug 22, 2019 11:33 am

Hello all, this is my first post but I have been lurking for a long time. I have read probably 90% of all threads related to reloading subsonic 300blk, and none of the suggestions have been working for me.

I have an 8.3" unsuppressed barrel (muzzle brake) with a pistol-length gas system and a 0.0995" gas port (I think). I am using a normal carbine buffer and standard (read: cheap) BCG. Supersonic loads work very well.

I am reloading using the following recipe:
208gr Amax and 220gr ELD-X bullets
9.5gr up to 11.5gr Shooter's World Blackout powder (Lovex D063)
2.21" COAL
CCI #41 "Military" small rifle primers

No matter what I do, I can't get this to cycle properly (edit: not once has it actually fed the next round or locked back on empty). This powder comes highly recommended for this application (subsonic heavy bullets). Aside from increasing the gas port size or using a suppressor, is there anything else I can do to make this cycle? It seems others with similar setups are OK with this powder selection and loading.

Does anyone know the upper charge to stay subsonic with this powder in a short barrel? Most of the load data is with a 16" barrel.
Last edited by fancygunz on Fri Aug 23, 2019 7:25 am, edited 1 time in total.

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SJ 40
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by SJ 40 » Thu Aug 22, 2019 7:17 pm

I had simular set up 8.5" and found that changing out a std,carbine buffer for a H 2 buffer gave me complete function with all loads super and sub.

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bangbangping
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by bangbangping » Thu Aug 22, 2019 8:14 pm

fancygunz wrote:
Thu Aug 22, 2019 11:33 am
Does anyone know the upper charge to stay subsonic with this powder in a short barrel? Most of the load data is with a 16" barrel.
Nobody can answer that for you. I would think that 11.5 grains is fairly close, but without a chrono you're just guessing.

fancygunz
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by fancygunz » Thu Aug 22, 2019 9:37 pm

SJ 40 wrote:
Thu Aug 22, 2019 7:17 pm
I had simular set up 8.5" and found that changing out a std,carbine buffer for a H 2 buffer gave me complete function with all loads super and sub.
Isn't that counterintuitive? I always thought a heavier buffer meant more resistance to recoil and therefore should require more energy to lock back, no?

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dellet
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by dellet » Thu Aug 22, 2019 10:32 pm

A lot of things are counter intuitive with subs.

Does it cycle at all with those loads?

My first thought is a problem with the gun. I would check for gas leaks or restrictions. Make sure the BCG is moving freely in the upper. If the rifle is correct, no reason that load should not work. Port size is fine.

This is one of the reasons that with subs you start with the high end of listed sub sonic loads and work down. I would chronograph a load of 12.5 grains @ 2.250” with both bullets.

If if does not lock back on empty with the above loads, inspect the gas system. Make sure the trigger springs are in correctly, make sure the bolt face is about 1/4” behind the catch when full retracted.

Make sure the bolt is well lubed in the carrier.

That would be a start.
300 Blackout, not just for sub-sonics.

savage308
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by savage308 » Thu Aug 22, 2019 11:15 pm

when you say "cycle properly" we need more info. I have a 10" upper and some of my loads fire and feed good but they wont hold back on an empty mag. they are very accurate so I use them . I have one load that works in all my uppers. 220 berrys plated ,9.5 ALLIANT 2400, win sr primer, lc16 brass .

fancygunz
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by fancygunz » Fri Aug 23, 2019 7:24 am

savage308 wrote:
Thu Aug 22, 2019 11:15 pm
when you say "cycle properly" we need more info.
By not cycling properly, I mean it turns my semiauto into a bolt action. It does not feed the next round at all.
dellet wrote:
Thu Aug 22, 2019 10:32 pm
A lot of things are counter intuitive with subs.

Does it cycle at all with those loads?

...

If if does not lock back on empty with the above loads, inspect the gas system. Make sure the trigger springs are in correctly, make sure the bolt face is about 1/4” behind the catch when full retracted.

Make sure the bolt is well lubed in the carrier.
I will check the bolt and gas system and report back. This is actually my first DI AR, I only have experience with piston systems, so it's very possible that I'm missing something obvious...

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dellet
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by dellet » Fri Aug 23, 2019 8:20 am

Just so I’m not confused,

You have full function with supers, cycles and load locks back on empty every time.

Subs will eject the spent case, but not pick up the next round.
300 Blackout, not just for sub-sonics.

fancygunz
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by fancygunz » Fri Aug 23, 2019 10:38 am

dellet wrote:
Fri Aug 23, 2019 8:20 am
Just so I’m not confused,

You have full function with supers, cycles and load locks back on empty every time.

Subs will eject the spent case, but not pick up the next round.
Yes, exactly my problem, even at "max loads" to remain subsonic with a powder that seems highly recommended. I was thinking of trying a new powder, leaning towards CFE BLK or lil'gun.

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dellet
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Re: Subsonic Unsuppressed 300blk from 8.3" Barrel

Post by dellet » Fri Aug 23, 2019 11:29 am

Lil'gun will make the problem worse, CFE BLK might be able to mask whatever the actual problem is enough to work.

Pull the gas block, make sure there is no leak or blockage. A leak will show up as a carbon track. Make sure the port is centered in a carbon ring and measure it while it's apart. That is the most likely area of failure. Also check the gas key on the carrier for leaks or being loose.
300 Blackout, not just for sub-sonics.

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