Head Space?

[DaveB]

Okay background.....we have a new semi-auto AR kind of rifle in the house. Fired brass is showing indents where shoulder meets the base and they happen to match up exactly with the rifling. Also, case/shoulder is showing bright line as if turning in the chamber. We fired a total of 4 rounds. before the line was cold and we could venture forward and pick up brass.

I took one look at the very first fired brass and stopped all activity. I considered that perhaps the SB die had set the shoulder back too far but measurements at home say that is not the issue.

Eyeballing the BCG I can see where a cut has been machined directly opposite the extractor that is deeper than the face of the BCG and the depth of that cut is an exact match to the length of the indent on the brass.

I could use some help with this.

 

[10mm4me]

Pics would help, but directly opposite of the extractor should be the ejector which is a small post protruding from the bolt face and it is under spring tension.

 

[Mausermeister]

First: Please post pics of brass.

Is the upper a Bear Creek Arsenal? They are notorious for being over gassed.

Without pics of brass, my guess is over gassed and maybe a burr in chamber. The dented shoulder is from case hitting the deflector as it is ejected and brass going forward is a classic sign of over gassed. A bright line on brass sounds like maybe a burr in chamber is scratching it. The bolt twists slightly as it locks and unlocks in the lugs of the barrel extension and usually the brass turns with it. Indent on base of brass sounds like ejector pin striking the cartridge base. The more over gassed and/or hotter the load, the more noticeable it will be.

Try polishing the chamber, especially the neck with a drill and a small bit of very fine 0000 steel wool wrapped on a chamber brush and oil to get rid of the burr. A heavier buffer and/or recoil spring or adjustable gas block will help with the gas issue.

 

[EastTNHunter]

Pics? What cartridge?

 

[DaveB]

.308
We are shooting starting loads for 125 gr bullets.

Have acquired a box of factory ammo. Winchester power points. 150 gr.

Advertised velocity is or at least should be well withing CUP recommended limit for this semi-auto.

We are not seeing any indication of over-gas. The brass does not have that .223 dent .

The indentations in the brass shoulder are there whether or not the round is fired as is the scratch line on the brass at the shoulder.

The topmost fired round has the best pic of the indentations. They ARE NOT there on the unchambered brass. One of the unfired rounds chambered but a trigger pull did not fire them off. The other two were chambered and ejected. I keep thinking the marks are there as a result of the eject process but there is nothing visible for the brass to come in contact with. Every piece of brass has the marks but the fired ones the indents are deeper and longer.

 

[Omega]

Ok, here is something to look at, check your rifles headspace and match your brass' shoulder accordingly, and also check the area marked as "First Shoulder" on the pic for any debris. To me, it looks like gas has leaked from this area and became trapped between the chamber and the brass case, so when the pressure inside the case goes down, that gas indents the shoulder. The same can happen when sizing the case and lube gets between the die and case making a lube dent. A chamber cast may also be handy (or use a borescope), to see if one of the grooves didn't get into the freebore area making a channel for the gas to get into.

 

[DaveB]

These indents do not appear to be caused by a gas or material buildup or by manufacturing debris. They have sharp edges, like struck with a knife tip. However, the are small enough to be debris-caused and we have looked and looked and found nothing. And while they are small (there are at least 6 of them on each case) they absolutely should not be there. And additionally that does not explain the FTF issue.

By putting a dummy round-brass, primer, bullet exact SAAMI length-- into battery we get the same indents.

Whatever it is we are returning the rifle to the manufacturer.

And the line at the shoulder is something new to me-I have been loading since the mid-1980's and have never seen brass marked like that. And in nearly every caliber I own we are at max charge weights. Something is amiss so back it goes.

 

[Tenntrapper]

For what its worth, that looks like Nosler ammo. Few years ago, I bought 5 boxes of their trophy grade 308 accubonds. Maybe a 1/10 of them would fit my chamber (rem 700). Of those that you were able to close the bolt on, it was difficult.. and very inaccurate.
Seems I read here recently that someone else was having trouble with their 308 ammo.
Might want to try another brand.

Could also be a chamber that was cut to short.

 

[DaveB]

No factory ammo fired yet.
These are starting load values from Nosler/Hodgdon. Objective is to keep MV in 2750-2800 range.

The same exact loads fired in a bolt .308 are MOA. No indents.