I have some marks on my brass I can't identify and I'm trying to figure out if it's a problem or not.

.300 BO, Aero Precision stripped lower, BCM enhanced lower kit, BCM carbine buffer, Geissele SSA-E 2-stage trigger, Radian Ambi charge handle, Radian ambi mag release, Radian enhanced bolt catch/release, AR-Stoner 10.5" upper, Venom Vortex red dot, Magpul BUIS, Magpul .300 BO poly magazines, Blue Force Vickers 2-point sling. I use a Caldwell brass catcher but I fired some of each load without it installed and there's no difference.

I ran the troubleshooting checklist even though I didn't expect it to reveal anything because the gun runs fine as long as it isn't being fed 110-grain v-max.

AAC 110 grain V-Max (factory load).
Winchester 170-grain XP in Lake City brass (factory load).

All of the brass I fired through it, regardless of bullet or load, show two triangle shaped marks in the neck, in the same exact location. The marks are pretty consistent in depth. Many, but not all of them, have a ding on the lip of the case, often centered on the two divots. AAC and Lake City brass pictured show the same marks on all cases. I swapped out the BCG with a spare while trying, unsuccessfully, to get 110-grain v-Max run and the marks were the same after the swap. This post is not about trying to get the v-max to run in my gun. It won't; it's too short. I'd like to know what's causing these marks.

Second, on the Lake City brass one or two rings appear on the neck, near the lip, after firing. These marks are not consistent in depth. Again, I'd like to know what's causing this.

My reloading mentor, DryFlash3, looked at my pics and said he's never seen anything like it. When he was training me, he showed me a hundred or more cases with problems that he'd set aside for training purposes. None of them looked like these.

AAC on top, Lake City on bottom:



AAC brass, close-up showing the divots:



Lake City brass, showing the divots and the rings.



AAC Brass. Two on the left using the original BCG that came in the AR-Stoner upper. Two on the right using the Aero Precision BCG I swapped in. They look the same to me.



I made a grayscale of one of the Lake City brass. It's easier to see the detail of the marks, IMO.

Looks like feed ramp scoring? Or your lugs. I don't think its an issue for reloading purposes. Or firing.

Attached File

Yep scratches from the locking lugs as it's feeding. A different kind of magazine might change the feed angle slightly.

Dings are from feed ramps I'm guessing. You can see which way the material is being shoved. It will tell you if that's on insertion or extraction.

Rings are from crimp die.

Dents on neck are from deflector upon ejection.

Mystery solved (mystery to me, anyway). I'll be shooting with confidence today. Thanks one and all!