I am able to place a 390 barrel in a 391 receiver. I am able to place a 3901 barrel into a 391 receiver. However the issue is getting the forearms and gas pistons to workout.
I can't exchange my 391 forearms because the don't fit over the 390/3901 gas system. At the same point I can't get a 390 forearm to fit in a 391 receiver (larger/better square housing in 390 vs crappy little dinky lip in 391).
You would be able to swap out stocks and trigger groups as well... there are just issues with the change in forearms/gas systems.
Pat, I retro fitted your friends 303 barrel to fit a 390...not a big deal and there are 2 ways of doing that. The easy way is to just grind off part of the ring for the gas vent in the forearm, the other way (and far riskier) is to grind a portion of the cylinder support off of the barrel.
Anyway, any 391 barrel will fit any 391 (with the exception of the Xtrema, which has almost NOTHING in common with a 391 other than the name). You should use a Mobil barrel for a Mobil forearm, but you can use an Optima on a Mobil (it can be a bit floaty). A Mobil forearm can farely easily be retro fitted to an Optima barrel by relieving approx. .020" of wood to compensate for the thicker barrel.
Tron: I've got a Ballstics Specialties 391 coming with a 32-inch barrel. The 32-inch takes mobil choke tubes. I thought I would look around for a 28-inch 391 barrel that takes mobil tubes to use in sporting clays so I don't have to buy a new set of tubes. I just wanted to know if my new gun would handle the shorter barrel from an older 391.