Black Rifles 300 Blackout Upper on the way.. Adjustable gas port question

Couldn't resist the temptation to finally get a complete CMMG upper in 300 Blackout. I have a rifle barrel being made for my Savage Bolt Action, but my patience wore thin. (I always PLANNED to get the upper, I just moved the timeline forward.)

The package that I got only had the pistol length gas system, which is recommended best for sub-sonic loads. I don't know my future subs/supersonic shooting preferences, so I figured upgrading the gas block might be a little bit of an improvement on the stock gas sysem and allow me some adjustability incase something gets finicky.

Is this an accurate assumption, or should I just leave it alone for now? If it will benefit from the adjustable gas block, is there a decent model out there that won't break the bank?

Any pointers would be appreciated. I'm sure I'll have it at my next range outing.



Active Member
Re: 300 Blackout Upper on the way.. Adjustable gas port ques

You need to P.M. Clicker...He has a 300 Blackout and he did a lot of digging and reading before he got his built....He also has at least 1 adjustable gas system, but, don't believe it is on his Blackout....

Good luck with it..



Re: 300 Blackout Upper on the way.. Adjustable gas port ques

We need a bit more info like what length barrel and are you running a can?

Out of the four 300BLK owners I know & shoot with I think I'm the only one with an adjustable gas block - oh well here goes.

I have a PRI adjustable gas block on my 9 incher and so far it's worked well. I really shoot just subs since I use a pistol can that is not rated for use with supersonic ammo.

I replaced the Allen type set screw with a longer round head machine screw so it's easier to turn while shooting. Takes about 4 revolutions from closed to full open. I ground a notch into the screw head and painted the notch so I have a reference point while adjusting it. I put a few wraps of Teflon tape on it too.

The gas tube roll pin didn't fit very well and I could not get it to go completely through to the opposite side of the GB. It look like crap all splintered and shit but it works. I really need to redo it at some point just cause I know it's there!.

I've been playing with a bunch of reloads and the PRI GB lets me tweak that screw until the bolt locks back and then call it good.
Re: 300 Blackout Upper on the way.. Adjustable gas port ques


This one is the 16" upper, but has the pistol length gas tube. No can so far, and finishig up my trust paperwork before I SBR one of my lowers.

From CMMG's perspective, they seemed to have recommended the subsonic loadings for this upper, which is currently not a problem, as thats all I've stocked up on...

I'm assuming that the port might be larger for the pistol length upper, to get more flow.. although thats just a guess.. and a shorter tube would have less flow resistance as well.. Going supersonic would then seem to provide too much presssure - so to possibly compensate for any future use of supersonic loadings, or for finicky load/can interaction, I'd think at least having the adjustable block might provide one possible means to "tune" to some degree.

Its a theoretical premise, but right now, the set-up is supposed to favor suppressed or unsuppressed subsonic loadings. I might have that ack-bassward, but I'm using night-time logic.. :)

See the table below. The configuration I am getting will be the 16" barrel with the Pistol length Gas Block.. Not recommended for Supersonics based on this table.

I've got some factory loaded Subsonics, to test with, and NO supersonics are in the plans yet, about the only thing I'd possibly plan on shooting in it supersonic would be the 150 Grain Barnes 30/30 holllowpoint, I figured expansion would be pretty good in that load.




Re: 300 Blackout Upper on the way.. Adjustable gas port ques

OK I gotcha now. I think the issue with supers in that barrel is the dwell time. Too much gas entering the system with a pistol port location and a 16" barrel.
An adjustable block would be of use in achieving the most flexibility from that set up. The cost is the problem, I think the MSRP is $95, perhaps you can find a deal on a used one.
Paladin Machine makes a 3 position block but you can't really tune with it. I believe you must trim the gas tube when using this block.
I've heard that Delta Arms is working on some type of block but haven't seen anything from them yet.
Do you reload?
Re: 300 Blackout Upper on the way.. Adjustable gas port ques


I was replying via smart-phone so kept it brief earlier.. I did manage to find several 0.750 adjustable gas blocks on Midway for $49-99, so there is a variety.. I'll keep checking around and try to get one of those and maybe a free-floating handguard to change the look a little.

I don't reload, but am really wanting to, especially if I get back into hunting and shooting as much as I used to. With a fun plinking round - cheap/lower cost blackout loading, (and a can) I have a feeling my desire to shoot will go up exponentially.

I'd like to reload some of the higher-power cartridges I shoot as well, (7mm-08, 243, maybe the 260 or some other accurate 6 or 6.5mm round for hunting and to possibly play around with some target shooting)

After paying $22 a box for remington 300 Blackout, I know I definitely need to get some more lower-cost options if I want to do a mag dump-albeit a semi-auto mag dump.. LOL So either I need to invest in 5 round AR mags, or just begin angling my way towards reloading...

I've semi-stocked up in an earlier plan to get to reloading, and have some components already, but never set up a complete package and "bit the bullet" so-to-speak. I'm actually going back through my accumulated items and may try to work out a loading session/learning session with someone that does reload (preferably the 300 Blk) to decide if its for me before investing more.

I've got some 240 SMK's squirreled away and would probably be picking up some lower cost 200+ grain bullets and some converted 223-300 Blk brass to churn out some plinking rounds.

There's 2 bullets that have my interest for supersonic loadings, the 110 Gr Barnes TacX low speed, its made to expand at less then 2000 FPS, and the 150 Gr Barnes FB HP made for the 30/30, the frontal area of it should lead to decent expansion as well. I want to try the Subs on hogs, especially with the can, but for deer, I'll be weighing my preference for subsonic versus supersonic as of yet....

I have some of the 240 grain segmented bullets to test out as well..


Charles Edens