Took the .300 uppers out for the first time today!

[grey2112]

Had some issues, and hopefully you experts can help me out.

I have a 16" and 8" upper for my AR. Each barrel was done by a member here, with an adjustable gas block (High, Low, and Closed). The 16" upper has a DPMS upper receiver and the 8" has an ASA side-charging upper receiver.

I loaded up two different types of ammo.

1. 125 grain Nosler Ballistic Tip with 18.5 grains of H110 powder. I bought supposedly "ready to load" brass and so put the primers, powder, and bullets in without running them through my resizer dies. When I tried to dry load them they wouldn't extract, and so I pulled all the bullets, resized them, and then loaded them again.

2. 220 grain Sierra Match King with 9.6 grains of H110 (my subsonic load). These I DID resize before loading and never pulled the bullets.


When I got to the range I started off with the supersonic 125 grain rounds, but neither upper would eject the spent cartridge, regardless of the gas block setting. The case was simply stuck in the chamber - I had to use a cleaning rod to pop them out. In fact, some of the cases, when I removed then, had a 25% chunk (like 12 o'clock to 3 o'clock) of rim sheared off - like the extractor tried to pull the cartridge out but couldn't get it out since it was stuck so bad, but in the process sheared off that chunk of brass on the bottom of the rim. No problems chambering, though, and every one went BANG when the trigger was pulled, and the accuracy was nice.

I then tried the 220 grain subsonics - when set on LOW each round performed flawlessly, on both uppers. The HIGH setting exhibited similar problems as the 125 grain rounds, and the CLOSED setting worked as expected (didn't eject, but didn't get stuck). Of note is that when I put on my 9mm suppressor both uppers on LOW worked extremely well (and the subsonics were blissfully quiet).

So, WTF? Why can't I fire supersonic rounds without the cases sticking and occasionally having 25% of the base of the rim sheer off? And yet the subsonics work like champs? Could it be the bullet pulling and resizing? Perhaps I didn't clean off all the resizing lube on the 125 grain rounds and they got all sticky in the chamber?

Honestly, I got these uppers for subsonic work, and if I can't use 125 grain rounds for some reason I won't be terribly upset, but for some applications it would be nice to have the higher velocities.

[LouBoyd]

My impression from your post is that you're getting far too much gas though the system on the full power loads and it's trying to open the bolt early. The symptoms point to the gas port being either too close to the chamber or too large or both. I presume the gas ports are in the "pistol" location. It appears to me that the bolt is opening before the pressure has dropped in the chamber so the cartridge is locked by pressure in the camber and the extractor rips the rim off the cartridge. I expect the hard opening is from mechanical distortion that's holding the empty case in the chamber. Since both cartidges shoot ok with gas system closed it's not a problem with the chamber or the loads. Quickload indicates both of your loads give reasonable pressure and velocity.

Solution? I don't know how your gas block is built. Maybe making a new valve with both the high and low ports drilled smaller would make it work properly with both cartridges. Or maybe use a carbine or full length gas tube wrapped around the barrel before it goes to the bolt carrier to delay and slow the propellant gas. The gas port itself could be reduced in size with a sleve but that seems like a difficult task to do right. Its too late now but it's reasonable to start with a small gas port and test fire, then open it only if it's needed. That's true of the two ports in selector too, so the right ratio can be achieved. If it was mine I'd try a longer gas tube first. There is no benefit in trying to open the bolt early.