Putting together a 6mm ARC AR15 with an upper that I bought complete, used. No fire yet, I just test chambered and extracted some rounds. This is factory Hornady Black 105gr, I was surprised to see that the projectile had engaged the rifling this deeply. Is this normal? I assume that's what these marks are, or is there a potential problem here? I intend to start loading for this round and others, but as of yet have never loaded anything. I do have an OAL gauge to find "jam" but haven't yet gotten a modified case for this round.