4130 would be fine, I would stay away from 303 stainless. If you want to use stainless for gun parts use 17p4 prehardened or 416 stainless. They both machine great with carbide in the prehardened state. I would also use 4140 or 4130 prehardened also. Also 1144 stressproof seems to be used with great success. I prefer 4140 prehardened as it machines well, is easily blued, and has great wear properties. 4140 prehard is also very common in a lot of applications as well as firearms manufacturing so it is easy to find.