Thread: Trouble getting 300 Blackout to shoot and cycle subs

Hi There,

I am new to this forum and keen to get some help?

I have a CMMG 300blk upper with a 16" barrel, carbine gas system on a standard rockriver LAR - 15 lower. running a MAE 30'compact suppressor.

I am having trouble getting this to cycle subsonic rounds...and even shoot subsonic rounds, I have tried hand-loads(as below) and factory 208gn A-Max.

The hand-loads were 220gn sierra MK projectiles, small rifle federal primers, using ADI AR 2205 powder ranging from 9.0gn - 10.0gn I loaded in .1 increments, the only one that went sub was the 9.0gn the rest were super and non of them ejected. I am struggling to get the velocity low enough to be sub with enough gas pressure to eject the round.

208gn Factory load went sub and ejected but would not pick up the next round properly. (see attached)
for some reason the bolt was on top of the next round?????? is this Gas or a Mag issue?

I fired one round with the mag in and the bolt wont hold back so i'm assuming i'm not getting enough gas?

I have measured the gas hole in the barrel which is 0.112 of an inch, the inside diameter of the gas tube is 0.125, is it possible to drill the hole bigger in the barrel to give me more gas? what is the max diameter you can have without over pressuring?

can someone please help? not sure if I have a chamber/barrel issue causing not enough gas? or do I need to shorten my barrel and run a pistol gas system? I have also thought of changing to a lighter buffer spring but concerned how this would run on the super rounds?

I want my cake and to be able eat it as well, I want too shoot both sub and super while being suppressed on my 16 inch barrel.

Any help would be much appreciated!

Cheers,

Hadyn

Probably not what you want to hear but it could be a number of things.
I note you have brass rash behind the ejection port which could indicate ejector spring strength, extractor issues and or load issues.
I personally would start by changing powders.

Fault finding can be a huge process with a really simple soloution. Frustrating.
Plenty of people on here with experience in the subsonic department will chime in with better advice I am sure.

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Adjustable gas block would be the first installed if running subs.

Thanks, any suggestions on powder?

are you suggesting ejector spring strength, weak or strong?

would the adjustable gas block give more gas?

Originally Posted by Burgs12

would the adjustable gas block give more gas?

You can tune it to help the cycling, plus a suppressor on the end will increase pressure.

PS, tried other mags with the factory ammo?

Originally Posted by Burgs12

Thanks, any suggestions on powder?

are you suggesting ejector spring strength, weak or strong?

I don't think you have a spring strength issue. More of a timing issue due to how slow the carrier is due to gas.
As dogmatix suggested get an adjustable gas block first.
It will allow you to balance individual loads whether subs or full and might well be all you need to resolve the issue.



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Originally Posted by R93

I don't think you have a spring strength issue. More of a timing issue due to how slow the carrier is due to gas.
As dogmatix suggested get an adjustable gas block first.
It will allow you to balance individual loads whether subs or full and might well be all you need to resolve the issue.



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I would expect you might need to enlarge the gas port hole AND fit the adjustable gas block (so when you shoot the supersonics you don't "over-gas"), because if it wont cycle properly with all the gas it has now cuttinggas flow down wont help. ARs are generally over-gassed anyway for reliability

An adjustable gas block is used to sort out over gas issues and excessive bolt speed, especially with a suppressor and probably wont sort out your under gas problem with subs. A common problem with a suppressor on a gas impingement system is excessive gas blow back through the chamber and no amount of block adjustment can fix that, combining a properly proportioned suppressor is the answer for that issue.
In this case, as Mikee said, a bigger hole and then regulate with the adjustable block could be the answer.

Mathias' comments makes me thing that the gas hole in the barrel might even be too small to start with, ie a quality control matter.

Have a look at this link. Just breezed through it but it could be of help before you get the Black & Decker out.
300 BLK Port Size Testing

No such thing as over gassing with direct impingement as the carrier moves away from the tube venting the gas anyway.

You should have an adjustable gas block and pistol length or shorter gas system to reliably run both subs and supers.

Short of changing your gas system I would recommend increasing the gas port to the internal diameter of the tube. You can't get any more gas through than the diameter of that tube anyway.

Remember when testing subs to start high then work your way down. When you have your desired load fire a single round in the mag to see if the bolt locks back, this will indicate if you have sufficient gas to work the action.

Other fixes include removing one coil at a time from the buffer spring until this works and changing to a lighter buffer. Google h and h2 buffer weights to speed up or slow your bolt.

Good luck.

I run my 300BLK with both factory and home loads .. it cycles both supers and subs flawlessly ...
I use IMR4227 and 155gr pills or higher for sub's
Running a suppressor full time and have a 10.5" barrel with a "Pistol length" gas system(Non adjustable)
Im guessing your problem may be to do with where your gas port is situated in relation to the length of your barrel ... (maybe look this up online?)
Best of luck sorting your issues ... its a great cal to shoot

Get an adjustable gas block, it's not just a matter of under or over gassed.

Iv had lots of really strange things happen with the gas systems on ARs.

Also it's hard to trouble shoot without first being able to control the gas.