Put the ammo in a case gauge and verify spec first.

I’ll echo Sigfreund and say, “thank you, Rustpot!”

I don’t think I’ll get to take it apart again this evening, but tomorrow for sure. I’ll certainly keep you all posted (and likely ask more questions).

Check the star chamber for debris if you haven’t already. I had a blown primer end up in there somehow once that caused similar issues.

As far as the buffer spring is concerned, check the buffer tube and make sure its clean and smooth inside. The tube, spring and buffer are sometimes overlooked when cleaning an AR.

We opened it up again last night and found... yucky stuff.

I had too much lube on the BCG, and the inside of the receiver was littered with tiny brass pieces, a few of them in the barrel extension slots (grooves? lugs? what's the right term?).

So. All that's cleaned out, the BCG is cleaned and lightly lubed, the bolt has been cleaned. Hoping to get time this evening to check into that .025" gap business some more and check the inside of the buffer tube. Once all that's done, maybe I can shoot it some more Wednesday or Thursday, so as to have time to take it apart again if necessary over the weekend.

As a consumer, I'm bummed that I'm having this much hassle from a new upper. (I bought it a couple of years ago, but just put it to use a month or two ago.) Professionally, I'm in manufacturing, so I know "good enough" gets out the door sometimes. And as someone here said, "good enough is often neither good, nor enough."

I had an AR come to me that was sticking on insertion and extraction. The kid had cooked too much lube into a freaking goo in the chamber just mag dumping his FRT over and over.

It's something I was on the verge of creating a new thread about, but it may be relevant here...

You mentioned you're using Hornady ammo. I recently used their "SubX" 300BLK ammo, and was extremely disappointed. Accuracy was good, and function was normal, until there was enough garbage in the gun to cause it to...

...not go into battery.

You mentioned the inside of the gun was "littered with tiny brass pieces". That is exactly what I experienced. I think it's particulate that has something to do with whatever powder Hornady uses.

I would clean the gun and try different ammo. It seems highly likely, to me, that it's largely ammo-related. I doubt you'll see any of the troublesome "tiny brass pieces" with other ammo. I am confident it was that particulate that caused my problem.

I have shot plenty of Winchester subsonic "open tip range" with no issues. It is not accurate though. If you're wanting "performance" ammo for it, I recommend Nosler subsonic. I got some of the Nosler to try, after I experienced the problem with the Hornady. It is very accurate, and supposedly has good expansion at slow speeds, like Hornady advertises. Unlike the Hornady, it has no special garbage fouling up the inside.

Good luck!

Thank you, KSGM. I appreciate your chiming in on this one, as I'm certain your experience is far beyond mine.

In hindsight, I really wish I had taken a picture of the "glitter" inside that receiver last night before cleaning it out.

We've used some of the Winchester Super Suppressed (Mfr no. SUP300BLK) with results similar to yours -- not particularly accurate. (Then again, that could as likely be "vthoky error" as ammo problem.)

Midway may get some more of my dollars this week -- I see they have the Nosler 220-grain JHP in stock (#51275). Cant' find that stuff 'round here, that's for sure!

Nosler part number 61050 is what I have had a good experience with. Their other stuff will likely be equally good.

I put ten rounds of the Nosler 61050 into 3.5" at 100m with my 8.3" Ballistic Advantage barrel. The Winchester has about three times the dispersion at that range.

"Glitter" is a good description. I am confident that the ammo is a significant contributing factor to your trouble.

My experience with .300BLK is limited. My recent experience with the Hornady ammo just happened to coincide with your own.

Can you tell us the barrel length and gas system length? 300 BLK is almost impossible to get to function 100 shooting suppressed with subs without a pistol length gas system. It will always short stroke and not go back into battery.

I believe this barrel is 16”, with the carbine-length gas system. I’ll measure and verify in the morning.

Good tip, Crockett. I have never messed with a .300BLK other than a short barrel with a pistol-length gas tube, so I don't have that perspective.

How did this fail to come up before halfway through page two?!

Verified. Sixteen-inch barrel, carbine-length gas system.

As I understand it, the pistol-length system is about 3” shorter than carbine-length. Is that right? (Source)

I’ve only got about 2-1/4” “rearward” of the existing gas block, so I’m not sure how I’d fit a pistol-length system to this gun.



The diameter at the Allen wrench is .850”. The diameter at the pen is .700”, for what that’s worth.

This is the part where I admit — again - that I just don’t know enough. If I go to put a shorter gas system in place, it looks to me like I’ll need to change barrel, too. Right?

Looks pistol-length to me.

Does this pic help? I should have posted it a moment ago:




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KSGM, I believe you’re right.

I grabbed a known pistol-length upper and placed it beside this one… they match.

Have I got too much barrel for my gas system?

Thats a pistol gas system. Now that we know your set up for success you just need to get er dialed in.
Can you confirm which buffer is in your lower. IME with 16" barrel and pistol gas you need a standard weight buffer to get the gun to cycle. Can you confirm which buffer your using?

Also, I can't tell from the pic above, do you have a standard or adjustable gas block? While a wealth of info has been shared in this post, I think your problem is much simpler, IE just not getting enough gas. When this started did you ever fire enough to get an empty mag? If so did the bolt lock back?

Here’s the buffer and spring:





They’re nothing exotic — standard offerings from either Midway or Palmetto.

I did install an adjustable gas block a couple of weeks ago —> This one by Superlative Arms.

- - - - -

ETA: there are no marks on the buffer.

Does it have any markings on the face of the buffer? By face I mean the part that contacts your bcg. If there is nothing it should be a standard buffer. If there is an H or H1,2,3 it will be too heavy to run what you want reliably


****My bad, missed that you answered my question in your most recent post

DO you have the gas block turned open all the way? In my opinion an adjustable gas block just isn't needed for 300BLK and can cause more damage than good. I reserve them for gassy 556 guns