.308 ar - weird casing marks after firing

[speeddreamz]

Recently put together a Ma-Ten upper/lower with a fulton armory barrel and BCG (headspaced by them), and while adjusting my PRI gas block I noticed some weird brass markings. I am using some dynamit nobel 7.62 nato ammo.

The case has what appears to be 2 sets of perforations(like a perforations on your common pay stub) running vertically starting from the neck of the case. It only happens after shot, not when I load a round and eject it without firing.

If I can provide any more info please let me know.

[Robocop1051]

My guess is that you have two lines leftover from the machining process, in the throat of your barrel. When the bullet fires, the neck expands into those lines.

If I'm right (I'm just making an educated guess here) you have a pretty easy fix either send the barrel to a gunsmith or back to Fulton, and have the inside lapped. Send the casings with the barrel too.

[starfury]

Had the same issue with mine - the marks are coming from the sharp edges of the barrel locking lugs. When the case is extracted it drags against the ejection side (since the extractor is putting pressure on only one side of the case) – this is normal.  You can see this if you load a resized piece of brass (or snap cap) into the rifle and then slowly pull back on the charging handle while looking into the ejection port.    The edges of the lugs were very sharp on my new dpms lr308 (could really feel it with your finger).  I used a dremel tool and a polishing wheel (with a little jewels rouge) to dull them just a little along with polishing up the chamber.  You only have to do the two lugs on the extraction side.  Cleared up the marks on mine immediately. Edit* I found this old picture that shows the marks on the necks (before I touched up the lugs).

[survivalshop]

Had the same issue with mine - the marks are coming from the sharp edges of the barrel locking lugs. When the case is extracted it drags against the ejection side (since the extractor is putting pressure on only one side of the case) – this is normal.  You can see this if you load a resized piece of brass (or snap cap) into the rifle and then slowly pull back on the charging handle while looking into the ejection port.    The edges of the lugs were very sharp on my new dpms lr308 (could really feel it with your finger).  I used a dremel tool and a polishing wheel (with a little jewels rouge) to dull them just a little along with polishing up the chamber.  You only have to do the two lugs on the extraction side.  Cleared up the marks on mine immediately.

I agree . I wouldn't worry about ,unless its splitting the case . It may even clear its self up with a higher round count with the rifle .

If you suspect as Robocop said , its easy enough to get enough light at the end of the barrel & inspect the chamber for marks .

[hobbesgunner]

My DPMS did same thing.....little fit and polish on the lugs in barrel extension took care of it..

[Robocop1051]

I'll bet starfury and survialshop are more right than I am.

Put another 100-200 rounds through the rifle and I bet things clear up. Polishing the chamber is a quick do it yourself job too, especially if you have a rotary tool.

[Big Bore]

As the others said, normal.  The reason you do not see it with a loaded round being extracted is the bullet still in the case keeps the neck off the locking lugs.  Try it with a case with no bullet in it and I bet you will see the same marks appear.  On mine, I just shoot them until they don't appear any more.  Problem solved.  Those scratches are not deep enough either to hurt the case life, or at least none of mine have had their case life shortened by those scratches.