Home Forums Hunting & Shooting AR and Tactical Rifles I need Help!(suppressor/gas issue...)

Gentlemen, let me start by stating that I am an AR novice. I've had one(or two) for years but never really got into them enough to know anything about how they work, especially in the suppressed form.


That said I recently ordered a CMMG Dissent in 300 BLK and last Friday it showed up at my FFL guy. Friday evening I got it setup to shoot and yesterday afternoon I took it out for a test run.

First shot was with a CGS Hyperian K and Federal subsonic 220's. The second shot wasn't happening as the 'bolt' got 3/4 into battery and stopped. I pulled the charging handle back and the shoulders of the unfired cartridge had little indentations in them and the chamber was filthy dirty with gritty residue.


Cleaned the gun as best I could and repeat the same scenario. Cleaned it again and pulled the can and it shot okay. Action was still gritty but it functioned 90% of the time for about 30 rounds. The 'bolt' would hangup every so often but I think it was just because the action was still dirty.


I am thinking I need to open up the adjustable gas block to help the action work better suppressed but the amount of gritty residue was insane.

Several youtube vids show guys shooting these 'pistols' suppressed without issue so I'm wondering what in the hell is my problem?

Any ideas and advice would be much appreciated, thanks.

Adjustable gas block will surely help keep things cleaner, but they can be touchy with subsonics.

Barry, I've read that it's better to be over gassed versus under gassed for positive action function but by opening up the gas block screw it seems like that would make the rifle run even dirtier or do I have it backwards?


I need a long stem 5/64th allen wrench to adjust the screw and of course I couldn't find one in town so had to order.

Yeah, the more gas you have, the better the action will function... (if no can is used and the factory block works, which yours appears to be working fine without the can)

But the can will add lots of back pressure, so you can adjust to function using less gas with the adjustable block.

There's guys here that know a lot more about that stuff than me. I have a piston rifle with an adjustable gas block that doesn't let anything gas or powder foul back into the action/bolt area. Got friends that shoot gas w/ cans though, and we've had to combine wits to get them functioning cleanly... and I use "cleanly" meaning less dirty.

Okay, that makes sense.

What buffer weight and spring are you using? Are these factory or reloads? If reloads, lots of powders work well for supers, but only a few work well with subs.

I don't have an adjustable gas block, so I use a buffer and spring that gives reliable service with my subs, but is not over gassed for my supers.

When you shoot supers, where is the brass being ejected?

Factory Federal subsonic 220 grain ammo.

This is a funky pistol design, no rear spring.

[quote=SamOlson]Factory Federal subsonic 220 grain ammo.

This is a funky pistol design, no rear spring.



OK, don't know anything about those. I was able to play with buffers and springs to get reliable, clean service with subs and supers. If the gas block is all you have for adjustment, I'd start wide open for the subs and adjust down as needed. The suppressor should add back pressure and make it more reliable and in theory need less gas.
Maybe an issue with those factory loads?

Why not give CMMG a call. My br-in-law bought a 300bo unsuppressed pistol from Gunfighter Tactical and had similar issues with cycling. He didn't know much about ARs at the time and gave it to me to troubleshoot. I ended up calling GunTac. explaining issues. They had me pull off gas block and give them ID on it. He said "sorry we installed wrong gas block". They had installed suppressed gb with smaller gas port. Sent correct and it runs great now.
You mention "gritty residue" and that seems concerning. I've had dirty but never gritty with my ARs.

I just sent CMMG an email describing the issue.

Did some ammo research and found that subsonic ammo uses some type of filler in with the gunpowder? Evidentially that is the 'grit' that blew back through the gas tube.

Ordered some different ammo as well. It'll work out one way or another, thanks for the ideas.

Yikes. Time to load your own. The case is small enough not to need fillers.

no need for fillers is right. Unfortunately with factory ammo you often get what you get. I've tried small charges of even red dot with and without kapok.. Can see no difference. FWIW.

for loading your own subsonics and running a suppressor 1680 is your best friend.
reloader number 7 is very good also. shooters world Blackout is very close to 1680 and works great with some Sonic loads also. VV110 for me it's been the borderline it won't function correctly without the suppressor on but with a suppressor it does.

Powder availability varies, but I can get 1680 pretty easy, so that's my go to for subsonic loads. I use Lil Gun for my supers, which are primarily 125-135gr bullets. I've read issues about 1680 being dirty, but I really haven't noticed. I'm also not running a thousand rounds at a time either.

running suppressed I've not found anything that I would qualify as clean... I run my supers (125gr)with h110/296

Okay, here's a quick update.

I emailed CMMG and got great feedback, they were prompt and helpful.

I sent these pics to the them and they said that the little sand like particles are unburned powder. I had thought the same thing until I read about the filler that is supposedly used in some subsonic ammo.

They also recommended turning the gas screw out a couple clicks. The tech said that when running subsonic ammo even with the can there just isn't enough pressure to begin with to properly cycle the action. This is were I get confused, it would run better without the can than with and you would think there would be more back pressure with the can.

I just emailed them back and asked about that as i was busy working earlier and didn't think about it.

This is after one shot.

Maybe this isn't even dirty but I thought so.

this is awesome I think they'll take care of you this [bleep] is made right here in my own state.. actually know a few people in that factory.. they will take care of you if they do not let me know I will pull what little person things I have

Oh yeah man, I ain't worried.


It'll be a badass little gun when I get it figured out.

Chuck that piece of schit into the Missouri and screw your can onto a 223. Even if a subsonic 300 Blackout is running correctly it's still a rainbow trajectory turd and you might as well have a 22 LR.

Put some fire to that residue and see if, or how, it burns.

Shouldn't be hard to tell if it's powder or filler.