Subs in a suppressed SBR won't cycle

[DanVoils]

As the subject title says I have an SBR that I'd like to shoot suppressed subsonic rounds out of but it won't cycle. It fires the first round and any manually cycled rounds with zero problems.
Specs:
10.5" 5.56 barrel,
standard weight/size buffer and spring,
AAC M4 2000 suppressor,
AAC 51T Flash Hider with suppressor quick connect,
.223 77gr 1,050 fps cartridges,
.223 55gr 1,000 fps cartridges.

I've lubed and polished all of the moving parts and it has less than 500 total rounds through it.

With supersonic it functions fine both suppressed and non-suppressed. With both of the subsonic it will not cycle suppressed or non-suppressed.
The "gas to the face" is non-existent with sub-sonic and bearable with super sonic. I have swapped out BCG's, charging handle, buffer springs, buffers and complete uppers to no avail. (I have a second 10.5" 5.56 upper.) I have not tried a different lower as I was running out of time and patience. The gas block is a standard low profile that has been tuned for maximum gas transfer. (Plug the breech, blow down the barrel and move the gas block until the pressure pushing the air into the barrel is minimized thereby making sure the barrel and gas block holes are fully aligned.)

Is a 10.5" barrel too short for suppressed subs? Do I need to go with a 16"? The purpose of this is a bedside HD gun that wouldn't blow my ear drums out if fired indoors. I like the short compact length of the 10.5" with the suppressor. A 16" might be too long but if it's my only option I'll deal with it.

Thanks,
Dan

 

[rvb]

AAC M4 2000 suppressor,
The purpose of this is a bedside HD gun that wouldn't blow my ear drums out if fired indoors.

So, um, why do you need/want subsonic ammo?
full power ammo w/ the can won't blow your ears out. I shoot mine w/o ear pro all the time.
And you're going to trust subsonic .223 for HD? might as well stab them with a pencil...

can't help w/ your actual question w/o looking at load data. you may need to look for even slower powders to help generate more gas. may also need to open up the gas port.

-rvb

 

[rvb]

The gas block is a standard low profile that has been tuned for maximum gas transfer. (Plug the breech, blow down the barrel and move the gas block until the pressure pushing the air into the barrel is minimized thereby making sure the barrel and gas block holes are fully aligned.)

that's just a horrible way to align a gas block...
but you may need to open the barrel port regardless, assuming everything really is lined up.

-rvb

 

[Cameramonkey]

So, um, why do you need/want subsonic ammo?
full power ammo w/ the can won't blow your ears out. I shoot mine w/o ear pro all the time.
And you're going to trust subsonic .223 for HD? might as well stab them with a pencil...

can't help w/ your actual question w/o looking at load data. you may need to look for even slower powders to help generate more gas. may also need to open up the gas port.

-rvb

Im no expert, but I believe its to not also have the supersonic "crack" of the round as it goes downrange. Whats the point of making the boom quieter if you still have a loud secondary noise?

 

[DanVoils]

that's just a horrible way to align a gas block...
but you may need to open the barrel port regardless, assuming everything really is lined up.

-rvb

How do you suggest maximizing gas transfer then? If you have a better way I would love to hear it.
Dan

 

[DanVoils]

So, um, why do you need/want subsonic ammo?
full power ammo w/ the can won't blow your ears out. I shoot mine w/o ear pro all the time.
And you're going to trust subsonic .223 for HD? might as well stab them with a pencil...

can't help w/ your actual question w/o looking at load data. you may need to look for even slower powders to help generate more gas. may also need to open up the gas port.

-rvb

I have shot super sonic through it suppressed many times outdoors without ear pro many times as well without any problems. Have you shot your within a house without ear pro? I would imagine that would be significantly louder and that is what I'm trying to reduce/eliminate.
Why wouldn't a 77 grain bullet at 1050 fps at close quarters be adequate? How is it the same as using a pencil? What do you use?

Dan

 

[DanVoils]

Im no expert, but I believe its to not also have the supersonic "crack" of the round as it goes downrange. Whats the point of making the boom quieter if you still have a loud secondary noise?

Precisely. Even though the distances are much shorter there is still that secondary crack. I can only presume RVB is looking to do a single kill shot at 500 yards, something that is not inline with a HD SBR.
Dan

 

[avboiler11]

Yeah...I don't think you are going to get subsonic 5.56 to cycle in an AR platform, even suppressed, without a seriously tuned gas port/block/BCG/buffer/spring combination.

I also wouldn't trust my life to subsonic 77gr match bullets; they won't expand or fragment one iota.

If this is the path you are going down, I would seriously consider a 300 Blackout.

 

[T.Lex]

Maybe its a really big house?

(Nothing substantive to offer, but interested in the answer.)

 

[rvb]

Im no expert, but I believe its to not also have the supersonic "crack" of the round as it goes downrange. Whats the point of making the boom quieter if you still have a loud secondary noise?

the sonic crack is NO where near as loud as the gases escaping the gun.

How do you suggest maximizing gas transfer then? If you have a better way I would love to hear it.
Dan

I take measurements, blueprint out the gas block, mark the center of the port on the barrel and block, align the ports that way, add feeler gauges as shims between the shoulder and block to get the right offset....

-rvb

 

[Cameramonkey]

the sonic crack is NO where near as loud as the gases escaping the gun.


-rvb

Correct. But if you were trying to quiet down a car's exhaust, would you put on a larger muffler while ignoring a pinhole in the exhaust manifold? It may not be as loud as what was coming out the pipe, but it is still there making an overall impact.

 

[rvb]

I have shot super sonic through it suppressed many times outdoors without ear pro many times as well without any problems. Have you shot your within a house without ear pro? I would imagine that would be significantly louder and that is what I'm trying to reduce/eliminate.
Why wouldn't a 77 grain bullet at 1050 fps at close quarters be adequate? How is it the same as using a pencil? What do you use?

Dan

77gr @ 1050 = not a whole lot more energy / power factory than a good .22LR. you can make those real quiet at least...
.223 needs velocity to fragment and/or expand to be viable as an HD round. you'd be much better off w/ 9mm or other pistol caliber (you can make 9mm really quiet and still get significantly better HD performance).

while not "indoors" I've shot mine in confined enough spaces to not be worried about it for a few rounds for HD. I'd much rather the rounds be effective, IMO.

I've also been downrange in the target pits w/ rounds passing about 3 feet overhead. it's just not that loud...

-rvb

 

[rvb]

Precisely. Even though the distances are much shorter there is still that secondary crack. I can only presume RVB is looking to do a single kill shot at 500 yards, something that is not inline with a HD SBR.
Dan

ok

 

[rvb]

Correct. But if you were trying to quiet down a car's exhaust, would you put on a larger muffler while ignoring a pinhole in the exhaust manifold? It may not be as loud as what was coming out the pipe, but it is still there making an overall impact.

Is the goal to be super secret squirrel stealth or not blow out our ear drums??? don't move the goal post.
-rvb

 

[rvb]

btw, I hope to get into a shoot house this weekend @ the honor flight charity match with my silenced/sbr'd HD gun. If I get to, I'll report back.
-rvb

 

[DanVoils]

Yeah...I don't think you are going to get subsonic 5.56 to cycle in an AR platform, even suppressed, without a seriously tuned gas port/block/BCG/buffer/spring combination.

I also wouldn't trust my life to subsonic 77gr match bullets; they won't expand or fragment one iota.

If this is the path you are going down, I would seriously consider a 300 Blackout.

You're the first one to even mention a resolution. Thank you! Now the question is how to tune said components.
I agree on the expansion of the 77gr. My thoughts were multiple holes would probably work pretty well. A 30 or 40 round mag seems to be a good way to start.
I have an 8" .300 BLK with suppressor. I need to get some subsonic rounds for it as all I've ran through it thus far are supersonic.

For those who think this SBR will end up in Sand-Box-Izstan it's not. It's going to sit between the bed and the night stand that has a 9mm siting on it in condition 0. If nothing else this is an exercise in function and functionality. Plus shooting suppressed sub-sonic is a great way to teach people how to shoot. Having them not anticipating the cracks reduces flinching.

Dan

 

[DanVoils]

btw, I hope to get into a shoot house this weekend @ the honor flight charity match with my silenced/sbr'd HD gun. If I get to, I'll report back.
-rvb

Will you be shooting it without ear pro?
Dan

 

[rvb]

you may need to look for even slower powders to help generate more gas. may also need to open up the gas port.
-rvb

but you may need to open the barrel port regardless, assuming everything really is lined up.
-rvb

You're the first one to even mention a resolution. Thank you!

so my suggestions don't count since you didn't like that I questioned the premise?

Will you be shooting it without ear pro?
Dan

hope so, if that flies w/ their rules.
-rvb

 

[rvb]

I have swapped out BCG's, charging handle, buffer springs, buffers and complete uppers to no avail.

CH has nothing to do with it. swapped out what for what? tried a light weight BCG? lighter buffers? That's all a bandaid for a gas problem though, you'll need more of it. you're probably only running < 1/2 psi at the port. pistol gas system may be only real option.
-rvb

 

[avboiler11]

You're the first one to even mention a resolution. Thank you! Now the question is how to tune said components.

I can't think of any way to set a rifle up so that it fires both subsonic 77gr and supers of any weight. You might be able to get away with a low-mass BCG, carbine buffer and light spring in a lower...but such an upper would have to have a big-ass gas port and an adjustable gas block with a wide range of settings. You'd also have to remember to reset the AGB before shooting supers, lest you damage the weapon.

I agree on the expansion of the 77gr. My thoughts were multiple holes would probably work pretty well. A 30 or 40 round mag seems to be a good way to start.

You really don't gain much doing that, IMO; yeah you perforate them a lot, but you'd have a fair bit less energy than a .380, with a bullet 2/3 the diameter and zero/zip/nada expansion.

I mean, you could use a 10/22 or Charger with a can and 50rd drum full of Mini-Mags and basically accomplish the same thing more reliably for less money.

Or, grab the 300 AAC upper, the suppressor, a few boxes of Nosler Noveske 220gr (subsonic *and* expands!) and make it turnkey.