Have 3 uppers I'm working on.

.300 whisper

6.8 spc

7.62x39

 

I have to find brakes for them. Am I wrong guessing the 6.8 and 7.62 will use .308 brakes. The .300 possibly too?

You are correct, 7.62 brakes are what you want.

For future reference

 

.300 whisper and blackout are the same

 

6.8 and 7.62x39 use a .308 brake

 

I learned the hard way and will be listing a couple .300 brakes