Thanks so much for your reply it is in deed a F prefix. the barrel nut is 7/8" front to back. I have been wanting to build another one as I am afraid to mess with this one, I am afraid if I swapped barrels back and forth that I couldn't get this setup back the way it is now. (Which is about perfect). If I was confident I would not mess it up I could/would, restock it and get the other caliber of choice barrel. It would also save me some cash and safe space. I am mechanically inclined and have read through HOW to do it, not intimidated at all with that part. I guess it is just hard for me to mess with something that is this close to perfect. I know some of you guys have been where I am now so all advise is appreciated. It still has the plastic stock and is not a beauty queen.