300 Blackout AR Pistol Cycling issue

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Hey all,

I'm curious about people's expectations for 300 Blackout AR pistols and cycling various rounds of different weights...

Here's the basic history:

Built my own lower (functions great, no issues)
Bought a fully assembled 300 Blackout upper with a 10.5" barrel and carbine length gas system.
(Side note: Me and 3 friends all bought the same uppers and built lowers)
At my house, we tried all 4 uppers and got these results:
  • 150 grain: 3 of 4 uppers would cycle properly (the 4th cycled most of the time); short stroking
  • 220 grain (subsonic): 4 of 4 wouldn't cycle properly
Sent them back to manufacturer (they were very nice, paid shipping, etc)...
It was suggested that they should have probably been pistol length gas systems, so we waited quite a while for the 10.5" uppers with pistol length systems to come in stock.

Waited a long time (though they kept me updated and communicated status well). Eventually, is seems like they weren't going to come in stock any time soon, so they suggested a 7.5" barreled upper with a pistol length gas system. I did quite a bit of research, and it seemed that, on average, due to typical 300 AAC powder burn rate, you'd lost maybe 200 FPS on the super sonic rounds and only about 50-60 FPS on the subsonic rounds, so I decide to give that a try. Here's the results from the next round of testing with this new option (only tested with one upper so far, instead of shipping all of them back and forth):
  • 125 grain: Cycled fine (with or without suppressor)
  • 150 grain: Cycled fine (with or without suppressor)
  • 208 grain (subsonic): Wouldn't cycle at all (still short stroking, suppressor or not)
  • 220 grain (subsonic): Cycled fine with suppressor, but still short-stroked without it
So... I'm curious. What would be your expectations for a situation like this? Should an upper like this be able to cycle everything form 125 grain to 220 grain, regardless of being suppressed or not? I'd think so, but not 100% sure. I was tempted to say "good enough" when 125 & 150 worked and 220 grain worked when suppressed (since I'd only use subsonic because I had a suppressor on), but the 220 gain are just FMJ for practice, so I was kind of bummed when the 208 grain didn't work, even when suppressed, as I'd be more likely to use the 208 grain AMAX rounds for home defense.

Thoughts and/or suggestions?
 

matts

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If you want to disassemble the handguard on it and remove the gas block i would be interested in the gas port size in the barrel.
This has a big affect. Many forums on it to read . I have personal experience and have had to open the gas port size two times to get it right, on my 16" carbine length setup.

Only other quick question i have is are you trying to run the BCG "dry" some folks don't like using lubricant, "messy", they will have cycling issues.
 

Reno

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Your experiences are pretty typical. The pressure differences between full power supers and what is essentially 45acp-like subsonic loads is quite substantial. Getting a gun to cycle all of those loads without being extremely overgassed shooting full power ammos is difficult.

If I did up a 300 black, I’d get a larger gas ported barrel and an adjustable gas block.

That or I’d build one strictly for subsonic ammo.
 

ELRShooter

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I am going through this as we speak.

watch the ejection angle of the shell, that will help tell you the problem. I saw mine change as I have moved up buffer weight and spring strength.

I started with a standard buffer. I now have the red spring from springco and a 5.6oz buffer.
110’s run fine, but 150’s won’t cycle.

just got a SLR 7 adjustable block today and expect that will fix the problem.

I am running a 10.3” BA barrel.
 
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Adjustable gas blocks and changing the buffer weights gets 'em almost every time. What were you running for buffers and springs?
what he said... and
it is recommended an H2 buffer for a 8.5" 300 BLK barrel for subs with no suppressor H2 is 4.6 oz


i am running an H2 in an 8.5 with bird cage and it cycles supers and subs just fine
 
OP
Skier
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If you want to disassemble the handguard on it and remove the gas block i would be interested in the gas port size in the barrel.
I'm trying to avoid that for now, as I'm working with the manufacturer to try and have them get it right, because whatever needs to be done, needs to be done on 4 of these (friends bought these uppers/kits together and got a discount on a group buy :) ). It seems like it might be a challenge to measure a gas port... Do you use various drill bit sizes to see what fits. I have a micrometer, but I'm pretty sure the hole would be too small to measure with it.

This has a big affect. Many forums on it to read . I have personal experience and have had to open the gas port size two times to get it right, on my 16" carbine length setup.
That makes sense, and will be an option if needed, but at this point, trying to try things that are reversible (in case it's NOT the issue).

Only other quick question i have is are you trying to run the BCG "dry" some folks don't like using lubricant, "messy", they will have cycling issues.

I've not tried any lubricant at all just yet, other that anything that might have come on it from the manufacturer. The BCG is nitride, feels nice and smooth (not fouled up yet, as it's probably only had about 40 rounds through it so far), but it definitely doesn't feel greasy, oily, or otherwise wet. I'd think it should work relatively dry when fairly new, but I guess a little lube would be an easy thing to try. It would be nice to know that it's not needed to function.

I also kind of wonder if maybe the spring would "break in" after a few more rounds or if they pretty much stay about the same for many, many rounds...
 
OP
Skier
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Your experiences are pretty typical. The pressure differences between full power supers and what is essentially 45acp-like subsonic loads is quite substantial. Getting a gun to cycle all of those loads without being extremely overgassed shooting full power ammos is difficult.
That makes sense, but it seems a fair number of people are able to run them, but maybe they're just way over-gassed when running supersonic and just don't realize it.

If I did up a 300 black, I’d get a larger gas ported barrel and an adjustable gas block.

That or I’d build one strictly for subsonic ammo.
I'm leaning toward this build being primarily for supressed, subsonic ammo, so I'd be okay with it functioning well under those conditions and at least not blasting itself apart when running supers. ;-)
 
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I'm trying to avoid that for now, as I'm working with the manufacturer to try and have them get it right, because whatever needs to be done, needs to be done on 4 of these (friends bought these uppers/kits together and got a discount on a group buy :) ). It seems like it might be a challenge to measure a gas port... Do you use various drill bit sizes to see what fits. I have a micrometer, but I'm pretty sure the hole would be too small to measure with it.



That makes sense, and will be an option if needed, but at this point, trying to try things that are reversible (in case it's NOT the issue).




I've not tried any lubricant at all just yet, other that anything that might have come on it from the manufacturer. The BCG is nitride, feels nice and smooth (not fouled up yet, as it's probably only had about 40 rounds through it so far), but it definitely doesn't feel greasy, oily, or otherwise wet. I'd think it should work relatively dry when fairly new, but I guess a little lube would be an easy thing to try. It would be nice to know that it's not needed to function.

I also kind of wonder if maybe the spring would "break in" after a few more rounds or if they pretty much stay about the same for many, many rounds...
Bore scope. See the holes on the screen in all their glory and you should be able to determine if there is an alignment issue that warrants some disassembly.
 
OP
Skier
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Adjustable gas blocks and changing the buffer weights gets 'em almost every time. What were you running for buffers and springs?
The spring seems to be a relatively standard carbine spring with about 37 coils (+/- 1 or 2, depending on how count them) and the buffer weighs in as a standard carbine buffer at 3 Oz. I thought about maybe trying a lighter buffer or spring, but not sure which would be the better to swap. I'd lean toward a lighter buffer before spring, but not 100% sure which would be better or if both would be needed.
 
OP
Skier
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I am going through this as we speak.

watch the ejection angle of the shell, that will help tell you the problem. I saw mine change as I have moved up buffer weight and spring strength.
Why are you going up in weight and strength? I was thinking I need to go down in weight, so there is less mass to move and thus easier to get the BCG to go back far enough to get behind the next round and strip it. From what I read, the buffer weight is the primary control of dwell time and the speed of the BCG coming back (as the sprint doesn't do much to counteract it during the short acceleration time (e.g. the force pushing backward is ~100x the power of the spring, so the buffer is what controls the unlock from battery time and reward movement velocity, and the spring controls the captured energy and return forward force/velocity).

I started with a standard buffer. I now have the red spring from springco and a 5.6oz buffer.
110’s run fine, but 150’s won’t cycle.
Yeah, that wouldn't surprise me with my experience so far. If I can't get 208 or 220 grain to cycle consistently, then I'd think an even heavier buffer/spring combo would push that bullet train threshold down even further.

Side note: I did a little more suppressed testing and found that the 220 gr subs didn't always cycle, even with the supressor. Recall from the original post, they never cycled without a suppressor, and they do sometimes cycle with extra back-pressure of a suppressor, but not always, and still the Hornady Black 208 grain Amax never cycle, suppressed or not.

just got a SLR 7 adjustable block today and expect that will fix the problem.

I am running a 10.3” BA barrel.
Let me know how that goes; I'm curious to hear. :)

I do kind of wonder if we'd been able to get a 10.5" barrel in a pistol length system, if that might have enough pressure to cycle properly. I'm betting then when we swapped our the 10.5" barrel with a carbine length gas system for a 7.5" barrel with a pistol length gas system, those two factors cancelled out (i.e. getting more gas sent back by pistol length gas system, but gas escaping barrel sooner, and thus pressure dropping sooner, on the 7.5" barrel).
 
OP
Skier
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Just curious, would a piston gun have these same problems? Seems like that would be the way to go? Plus no buffer tube means you can fold/collapse the brace to make a tiny package.
No buffer tube on a piston gun? Where does the BCG go? I've never worked with a piston gas system, so I might just be ignorant, but I'd think the BCG would still need to cycle and go back somewhere... No?
 
OP
Skier
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what he said... and
it is recommended an H2 buffer for a 8.5" 300 BLK barrel for subs with no suppressor H2 is 4.6 oz


i am running an H2 in an 8.5 with bird cage and it cycles supers and subs just fine
So you're proposing that a heavier buffer might help in a 7.5" 300 BLK barrel? That seems counter-intuitive. Other than the obvious experience that it seems to work you, any idea how that makes sense or works?

Maybe your gas port is larger, so your was more gassed than mine, I guess, but if I'm not getting enough reward motion to get the BCG behind the next round on the mag, it sure seems like a heavier buffer would make it even worse. Do you have a link to this recommendation, so I could read it and understand the setup they're using?
 
OP
Skier
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Bore scope. See the holes on the screen in all their glory and you should be able to determine if there is an alignment issue that warrants some disassembly.
You're thinking that maybe the gas block is offset from the gas port a bit, thus making the port effectively smaller? I guess that could make sense, but the same problem appeared across all four AR-pistols (which I guess is possible if the manufacturer is off in their process... at least they'd be consistent. hehe).
 

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