Gunsmithing M40a1

You could confirm with McMillan, as they should have the specs on hand, but I believe the exact details of an M40A1/A3 (if you just say M40 barrel, some might think you're referring to the original Vietnam era M40, which had a different contour) barrel is a Douglas #7 with a 4" cylinder. Not sure if the number nomenclature is the same between manufacturers.
 
ive seen mixed info....











looks like most sources are saying BO.....i dont know if some of the early M40a1s were Parked, or if the auction house is just mistaken.....
The early M40A1s that were built off Vietnam era M40 barreled actions probably just kept the original parkerized finish. Notice the green anodized Redfield scope and 40X base serialized to the action in the Rock Island auction. That leads me to believe the barreled action originally came off an M40. There are a few photos floating around of the same setup (early smear HTG stock with Redfield scope), and I think they were all either early production or test M40A1s, before the Unertls became standard.

So you could have a parkerized finish, but that would be more “correct” for early builds vice the standard issue M40A1 as it was during most of its life span, which had a black oxide finish.

But this is just nitpicking. Unless you had one of the rare shooting team or other “turn in” rifle, it’ll just be a clone or inspired build. Most folks wouldn’t even notice the difference between black oxide or flat black cerakote.