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Old February 21, 2020, 06:37 PM   #1
dyl
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Trouble shoot: 10.5 300 Blackout

Hey I made a pun.

I took out my 10.5" 300 blackout for the first time with Remington 220 grain subsonics. The bolt would not reciprocate far back enough to chamber another round or lock back on an empty magazine. Ejection was fine (not that it matters now).

I put this one together, it's my 3rd time assembling an AR, no problems during assembly. The barrel is a 10.5" Ballistic Advantage with a screwed and pinned gas block https://www.ballisticadvantage.com/1...ce-barrel.html(so I don't think I misaligned it due to the pin), and came with a pistol length gas tube. BCG/Bolt are NiBo Toolcraft. Buffer weight is carbine weight, and I'm assuming the spring is plain ol' carbine spring.

My next move is likely to
1) check the gas rings,
2)try a different buffer spring (not necessarily weight, but spring as my LPK was "budget")
3) then try lighter buffer weight,
4) then lighter buffer spring in that order.

Maybe different ammo, but I've never had factory subsonics fail to operate a firearm.

Do 300 blackout pistols usually require lighter buffer springs?
Thoughts?
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Old February 21, 2020, 09:09 PM   #2
9x19
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Silencer on board?

A pistol length gas tube on a 10.5" barrel may prove to be under-gassed if you're not running 'silenced'.
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Old February 21, 2020, 09:54 PM   #3
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As 9X19 stated your problem is the pistol length gas tube, you need to use a carbine length gas tube to be able to cycle your action on an SBR.
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Old February 21, 2020, 09:59 PM   #4
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I suppose you don't want to change your barrel to change your gas system, so I'd go with an adjustable weight buffer.

https://www.odinworks.com/Adjustable...r_p/os-abs.htm

Relatively cheap way to tune your gun.
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Old February 21, 2020, 11:21 PM   #5
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I *meant* to bring the Silencerco Hybrid but left it at home so I thought I’d at least sight in. So there are some factory loads that only function well suppressed? That’s surprising to me. I’d be okay with it, It’s just my first time seeing a factory subsonic load not work well unsuppressed. Im assuming it must be *that* much quieter when I do remember the suppressor.

That’s actually the only box of ammo I bought for it, I was planning on loading my own 220 grain reloads. Would some SuperSonics cycle the action better you think?
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Old February 21, 2020, 11:26 PM   #6
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A lot of 300 BOs are real close to overgassed on supers in order to lock back with subs. There is a lot more pressure and the pulse lasts longer on supers.
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Old February 22, 2020, 01:16 PM   #7
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Try some different ammo. I found the Remington 220 sub load to be pretty weak. It does run fine in my 9.5" pistol gas barrel (w/ and w/o suppressor), but the powder seems to have a slow-ish burn rate.

Make sure you don't have a rifle spring behind the buffer. If there are too many coils, it may not run well.
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Old February 22, 2020, 08:36 PM   #8
dyl
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Quote:
your problem is the pistol length gas tube, you need to use a carbine length gas tube to be able to cycle your action on an SBR.
I am not following, a pistol length moves the gas port closer to the point of ignition and allows more “runway” and a longer time for gases to be used to cycle the action before the bullet leaves the barrel. If anything I thought a pistol length has tube
is *supposed* to increase reliability in shorter barrels. Just not mine I guess.
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Old February 23, 2020, 12:44 AM   #9
ms6852
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Quote:
Originally Posted by dyl View Post
I am not following, a pistol length moves the gas port closer to the point of ignition and allows more “runway” and a longer time for gases to be used to cycle the action before the bullet leaves the barrel. If anything I thought a pistol length has tube
is *supposed* to increase reliability in shorter barrels. Just not mine I guess.
It becomes a timing issue and the lag time with the bolt will not be synchronized to allow time for the bolt to travel all the way back enough to allow the cartridge to be picked up in the magazine. If you replace the springs with the lighter coils the bolt may began to get damaged when it slams to the rear of the receiver housing.
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Old February 23, 2020, 03:12 AM   #10
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I bet the suppressor will provide enough additional back pressure to cycle the action.

I wouldn't get upset until you shoot it suppressed.
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Old February 23, 2020, 09:15 AM   #11
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Double check that it goes all the way back during hand cycling. Might be a spring issue.
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Old February 24, 2020, 12:58 AM   #12
dyl
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Quote:
Make sure you don't have a rifle spring behind the buffer.
Quote:
Double check that it goes all the way back during hand cycling. Might be a spring issue.
39 Coils in the buffer, which I google'd was correct for a carbine spring. I'm able to hand cycle the bolt and lock it back on an empty mag. I won't be able to get back to the range for a few days, but will definitely try with suppressor, and try get some hotter ammo.
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Old February 24, 2020, 10:25 AM   #13
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Keep us posted.
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Old February 24, 2020, 10:57 AM   #14
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Sounds like the right parts are there.

Did you do any clean-up on the parts, or lubricate?
I'm assuming everything is new.
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Old March 22, 2020, 10:36 AM   #15
dyl
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Update: 300 BLK pistol works! Question: Residue?

Hey all, long story short I got the 10.5" 300 Blackout Pistol Barrel w/ pinned gas block to WORK. I think it was a combination of: break in period and home reloads with insufficient powder.

Breaking in with light loads didn't fully cycle things and after ~20 rounds with 217 grain hard cast lead with decent powder charges everything worked, even the factory Remington subsonics that didn't fully cycle earlier. Each of the 217 grain reloads worked as well. At least, suppressed. I hadn't tried unsuppressed. I had started my reloads with extremely wide ACME 245 grain powder coated bullets that hung up a lot on the feeding and with very little data available I was too hesitant to charge with sufficient powder.

Now here's a question: I found powder residue underneath the handguard and on the gas tube just forward of the upper receiver. Is this normal because I'm shooting suppressed? I've checked my other AR's and there is no residue, even the 458 socom I've shot suppressed. Here's a picture

https://imgur.com/GY5RXuW
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Old March 22, 2020, 11:40 AM   #16
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Im guessing the source is forward of the receiver, not the receiver.
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Old March 22, 2020, 12:45 PM   #17
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Your carbon residue is probably from a leaky gas block. - Either between the barrel and gas block, or the gas tube and gas block.
With or without the suppressor, that will happen if it's leaking.
I have a couple uppers with minor leaks. No big deal, unless you're sticking your face or fingers over the leak.

Common knowledge says that carbon will plug the leak eventually. I'm not sure whether or not I believe that.
I, honestly, have never gone back and checked on some of my leaks, to see if they have diminished or stopped.
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Old March 22, 2020, 03:17 PM   #18
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I’d think that a pistol length gas system on a 10.5 in barrel would be more than sufficient.
I have a similar barrel, except I installed the gas block along with everything else and it functioned flawlessly on subs and supers ... but ‘twas with factory ammunition
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