How big is too big?

I recently got a new bbl for an AR platform build and after the first shot at range I noticed a frosted appearance of the body as well as pronounced bulging above the web of the case. I've attached a pic of fired case as well as a factory load.

I fired two boxes and two things were obvious, all the cases looked bad and had bulges above the web and the velocities were markedly less than expected.

I mic'ed some cases when I got back and found they averaged 0.0063" over chamber specs.

I don't know how much cases retract after they are fired so can only guess that the chamber might be another thousandth or more larger than that.

Saami drawings say chamber should be 0.378" above the web my cases are 0.3843"


Can anyone here tell me what would be acceptable? Do I adjust my charges and go or get barrel replaced?

I stopped completely after that session and put it away until I sort this out.

These are reloads?

You need to shoot factory ammo to troubleshoot.

If it does not do this with factory ammo, then there is something wrong with the reloads.


Frosted Casings are sometimes the result of a rough chamber.

Where did the brass come from?maybe someone cooked all the zinc out of them ( if you picked them up somewhere or heated them up red hot) Your chamber spec is how your chamber measures. if the barrel is used but new to you someone could have advanced a reamer in it to quickly and the chamber got crowded or it was rust damaged and then cleaned up or over polished.

I wasn't clear, I was using factory ammo from Federal.

What about your barrel you mentioned new? Brand new or a used replacement new to your build, more details would help. You could have a oval led out of spec chamber or its been mis-reamed. get someone to make a cast of it and then measure it. Have a look at the firing pin hit and see if its off center for any clues.

Brand new from factory.

I'm thinking if chamber needs polishing the dimensions will only get worse. SAAMI indicates dimensions are minimum with +0.002 tolerance so I'm already past that by o.oo43+

The chamber needs proved to help determine where the slack is coming from. maybe it (the barrel) got past inspection when it was being finished by being checked with the wrong component or test bolt. Im thinking out loud here but it could be a combination of things between the bolt carrier and bolt itself with a plus chamber. You need to eliminate some variables to determine where the slack is coming from. that's alot of slack in the head area and it could get ugly on you. Crimp your brass shut so it doesn't get mixed in with what you want to save but keep one or two to send with the barrel or keep with the rifle if you want. I would think that the possibility of a mis-reamed chamber would be less than the likely hood of other components that are not fit well but I am not an ar guru. There are alot of componant makers in the AR market that could tolerance stack up and show as problems. If your closing the bolt on minimum spec ammo in a plus chamber that brass is going to move somewhere and make a lot of bolt thrust that can wreck your rifle. I guess if the bolt has begun to unlock or is not fully locked before or after ignition the same thing may be possible with gas pressure still available to act against the case. Some one will surely jump in that can offer better advice. I would start by measuring my cartridges against the measurement I found from the chamber, dropping in a field gauge and then checking the bolt carrier and bolt against what I found.

I guess it's time to break out the cerrosafe.

If it is a New bbl in a New Firearm, doing that with quality factory ammo, the Mfg needs to deal with it under Warranty.

So one call to MFG and I was immediately promised a replacement and a refund of 15%. Additionally they are sending barrel along with a prepaid return label.

Maybe mine wasn't the first call.

Snav said:

So one call to MFG and I was immediately promised a replacement and a refund of 15%. Additionally they are sending barrel along with a prepaid return label.

Maybe mine wasn't the first call.

Click to expand...

Gotta love them happy endings! That's good news Snav.

Snav said:

So one call to MFG and I was immediately promised a replacement and a refund of 15%. Additionally they are sending barrel along with a prepaid return label.

Maybe mine wasn't the first call.

Click to expand...

It usually pays to take the time and make the effort to call.. good job!

Is it possible to have a chamber reamed large forward of the web? I would like to cast this chamber before I send it back but was told I might not be able to remove the casting.

I'm thinking the bulge is forward because the case head /web is thicker brass.

A chamber can be mis-reamed when the barrel is not running concentric or especially when it is done without a floating reamer holder. The reamer will stick to one side of the chamber so to speak because its spinning. crowding it with chips can make it worse. Warm the barrel up, plug the throat with a oil damp patch and make a cast. I wouldn't worry about it getting stuck with cerro safe. You can cast a chamber with sulpher powder from a drug store melted carefully and then pour it into the chamber. Its hard to do but the cast will be brittle and it may take a couple of tries so the other way is better. The smell is noxious too, lots of blue orange flames and cussing till you figure out how to do it. If they are going to replace it maybe you should just leave it alone or get it to some one who has done it before.