If you keep the pressure under the 9000psi it really makes no difference whether it is black or smokeless loads. In a 12ga it is pretty easy to get down around 6000psi - 7000psi. Helps to keep the shot load light, like one ounce, which will do just fine on squirrels. The age and the 12c with no other length marked next to it,like 70mm, would indicate it was originally chambered for 2 1/2" shells, 65mm, probably. Someone may have opened the chambers to the longer length after market however.