300 blk SBR won't hold open

[Bunk39]

Ok I've got a good one I think. My DD 300 blk SBR will hold open after the last round in the following conditions:

Subsonic suppressed
Subsonic unsuppressed
Supersonic unsuppressed

It will NOT hold open while shooting:

Supersonic suppressed

I'm running the factory gas block so I should be full port.

I'm running a JP heavy SCS buffer w the lightest spring.

For it to NOT hold open while shooting super's suppressed (highest pressure situation) my first thought is way under gassed. But it DOES hold open shooting super's UNsuppressed.

Am I over gassed and its cycling so fast that it doesn't have a chance to catch?


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[jdavid1]

May be stupid, but did you try several different mags or did you use just one?

 

[rcm192]

So it does cycle correctly? Maybe a lighter mass buffer would help. It seems like from you description the there is too much mass behind the bolt not allowing it to lock back. Try different mags first and go from there.

 

[JoeLiberty]

Adding a suppressor increases gas pressure and cyclic rate, so yes it could be over gassed. But if the bolt is outrunning the magazine follower, you would probably have feeding issues as well. Switching magazines is a good suggestion. I would say that 'trying different magazines' is to the AR-15 world is much like 'turning it off and back on again' is to the IT world.

 

[DAVE_M]

A heavier buffer is what you seek.

 

[JoeLiberty]

A heavier buffer is what you seek.

Or an adjustable gas block. If it turns out that too much gas is the problem, adjustable gas lets you turn that down directly, treating the cause instead of the symptoms.
Additional advantages would be reduced recoil and reduced gas to your face from shooting suppressed.

 

[DAVE_M]

Or an adjustable gas block. If it turns out that too much gas is the problem, adjustable gas lets you turn that down directly, treating the cause instead of the symptoms.
Additional advantages would be reduced recoil and reduced gas to your face from shooting suppressed.

A new buffer is cheaper and easier to replace, but an adjustable gas block would be the bees knees.

 

[Bunk39]

Thanks for the replies everyone. I've got an adjustable on another rifle and it is nice.

On this rifle I'm going to try a heavier buffer because I've already got the spring pack from JP. Hope I can find one that works all-around. If not I'll have to change the gas block.

Thanks again


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[DAVE_M]

Thanks for the replies everyone. I've got an adjustable on another rifle and it is nice.

On this rifle I'm going to try a heavier buffer because I've already got the spring pack from JP. Hope I can find one that works all-around. If not I'll have to change the gas block.

Thanks again


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The issue is the BCG is traveling too quickly and the buffer/spring can't handle it. It's traveling too quickly because it's overgassed when suppressed with supersonic loads. You have to remember that the buffer/spring/gas port size were all designed to work unsuppressed with supersonic loads. Even if you get the adjustable gas block, I'd still recommend getting a heavier buffer. Then you can tune it up real good

 

[Bunk39]

Well ran out back and tried every spring in the JP spring pack. Using the heaviest spring, 1 out of four brands of super's held open. So looks like an adjustable gas block is necessary.

The good news is that the subs still held open both suppressed and unsuppressed. So some combination of buffer and adjusted gas should provide reliable function with subs and super's with and without the can.


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[rcm192]

Well ran out back and tried every spring in the JP spring pack. Using the heaviest spring, 1 out of four brands of super's held open. So looks like an adjustable gas block is necessary.

The good news is that the subs still held open both suppressed and unsuppressed. So some combination of buffer and adjusted gas should provide reliable function with subs and super's with and without the can.


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My 556 sbr took alot of fine tunning especially with a proprietary buffer and spring setup to cycle reliably. I bet ill have to do it all over again when I build my 300 blk upper for it.

 

[Bunk39]

Yep. I was thinking the same thing. If I build/buy a 12" 556 upper, am I going to have to get a different buffer/SCS to go with it?

It never stops, does it


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[rcm192]

Yep. I was thinking the same thing. If I build/buy a 12" 556, am I going to have to get a different buffer/SCS to go with it.

It never stops, does it


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Never...thats why you end up with multiple SBRs and a hell of a stamp collection!

 

[Bunk39]

Yep. Going on six as we speak!


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[noob]

Yep. I was thinking the same thing. If I build/buy a 12" 556 upper, am I going to have to get a different buffer/SCS to go with it?

It never stops, does it


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I have a 12 inch noveske that runs fine on regular carbine spring and buffer. A couple 10.5 inch SBR's with h2 buffer but everything else standard.