4130 & 4140

[Green57]

G.T.

I would say stick with the 4140. This is the alloy that most modern fire arms are made from. It will machine easier without undue wear to cutting tools. The added carbon will allow it to be hardened effectively for the parts that require heat treat. It should take a salt blue finish without a problem as long as it is clean, and your bluing salts are at the proper temp.