40 s&w line on outside has anyone seen this before?

This line is on about 10 diff 40s&w brass caseing I have resized and cleaned. Has any one seen this before? Is it a stress mark ?

Federal brass has them. Good to go. I think some companies put it there to keep the bullets from sliding back into the case under recoil. Could be wrong about that though.

It's kinda of light black ring. I have seen those bullet set back rings on 45acp I have never seen them on 40s&w. good to know thank you !

^

I've reloaded hundreds of cases like that with zero issues

Thank you