It's the thickness on the back end. The tube doesn't screw in far enough for the scs to bottom out on the bump stop on it. I put the metal spacer in and the gas key can't hit the lower anymore. I didn't notice the mark until I switched from a carbine tube to the ubr. It doesn't bother me. The rest of the rifle is scratched and dinged. It got used lots and has traveled many miles and seen many rounds.
Also I'm running a suparms adj gb tuned to just lock back on an empty mag
Or simply drop a quarter in the rear of the buffer tube before sliding in the SCS. It's just the thickness needed. I know it's a little bit of a bubba method but that's what I did, simple and it worked.
Only problem is that it's getting pretty hard to find a spare quarter lying around these days with trudope at the helm of this sinking ship....
"Arguing with idiots is like playing chess with a pigeon..... No matter how good you are, the bird is going to s#!t on the board and strut around like it won anyway."