slayer, which gun are you running the 357SIG in? A Glock will have a more pronounced, almost flowing, primer strike on the 357SIG than any other caliber. In your case the primer edges are still round, but that's a weird looking firing pin mark. Actually looks very much like the strike marks from my 9mm JRC carbine. It's just the way that carbine runs. 124 Bayou TC on top of 3.4 grs TG at 1.100"
This is a flat primer...