I installed a PRI .308 muzzle brake on a 300 blackout I built but the shims are metallic in color and also larger than the barrel so they are visible and honestly look like shit! I'll see if I remember when I get home to take a photo and post it but I've been trying to do just that for several days and continue to forget so I decided to post without it.
What can I use in lieu of the shims so it is not visible and I get a better look and result, peel washer? Crush washer? Any ideas? Thanks!!!

I needed the larger ID shims to time a Gemtech brake on my .300.  Nobody had them in stock but Gemtech. They weren't larger than my barrel, but I didn't measure......

Just in case the barrel is a CMMG, I will try posting a picture this afternoon, you will see how sloppy it looks!!!!

If you're muzzle device will sit flush with the barrels shoulder and doesn't need to go very far in order to time correctly, I'd look into having the muzzle device faced enough to time.  I've done this with a couple of Battle Comps, an Ops Inc and a RAD brake.  As long as your facing job is square, you'll get a seamless fit.  I used sandpaper on a glass plate to face the muzzle devices.

I used a crush washer on mine. you could also paint the shims black.