With all due respect, why is it that a 30 inch barrel on a modern shotgun is considered " Long ", but on a muzzle loading shotgun, it is called " Too short???" I have an American Fowler with 30 inch barrel, 20 ga., and it swings properly, and has enough weight forward to hold when shooting round ball. The powder burns sufficiently to give me the velocities I want and need, so why the longer barrel? I think if he wants to cut 6 inches off that 42 inch barrel, the 36 inch barrel will be more than long enough for any use he will have. I would also do the work myself, first wrapping masking tape around the barrel where I want to cut it, then marking the length of the new barrel on the masking tape, so that I can guide the hacksaw through the barrel and stay on the line. Then a draw file to even up the end of the barrel, and finally some time spend with a hand drill and an old wooden file handle, chucked into the drill, with emery cloth in coarse to finer grits to taper the inside of the barrel so I don't cut patches on the rim when loading. Touch up the browning, or bluing, cut and shape the wood, and shorten the ramrod. Then solder, or have a machine shop, or gunsmith solder the front sight back on the new barrel length, and you are ready to go.
I hope he has already handled a gun with this length barrel, and likes how it feels and holds on target, before taking this much barrel off. If not, I would advise taking it down by inches, and testing the new barrel and stock before finalizing the length. It would be sad for him to cut this down to 36 inches, and then find out that 37 inches would work better for him. :shocked2:
Roger's rangers were known to shoot shortened Brown Besses because it was easier to reload them while kneeling in a canoe. For the kind of woods fighting they did, they didn't need a long barrel to make long shots they never had. It was 50 yds and under, and then hawk and knife to the death!