Shooting through a fairly new 6.5 creedmor semi, and noticed the brass is getting almost a triangle shaped dent. What is the cause of this? Y’all seen this before?

When the brass ejects, that's where it hits the deflector.

Brass deflector on an AR-15?

^^Yup

Size it, load it, shoot it again.

It'll buff out.

Thanks, everyone! Now that I am reloading and saving all my brass, I’m “seeing” this for the first time. Guess I didn’t pay much attention to my old .556 brass to see if that was happening to them...just tossed them in a bucket and saved em for my friend.

Which direction is the brass ejecting towards (i.e 1:00, 3:00, 5:00)? If forward of 3:00 you might be overgassed which is a bit harder on brass.

+1 on the chart above. Your brass is hitting shell deflector and getting beaten up. I had a grendel doing the same thing, just put in a heavier buffer/stronger spring I had lying around and problem solved. An adjustable gas block is probably the correct answer though.

I put half inch round stick on velcro, the "fuzzy" side on the brass deflector. Cuts the dents down to near never.

the dings are no big deal. The velcro option is common and does work. ideally adjustable gas block will stop this.

True, and can tune cycle to prevent other issues too. Good investment, was a must when I put a suppressor on.