steel slinger

I've read posts about replacing the recoil spring in the rear of the shotgun and replacing it with a stiffer spring, how do you remove the tube at the rear of the shotgun? I see it takes a 17mm wrench, when turning it does the end come off or does the whole tube come off at the back of the receiver? Thanks for any help on this proceedure.

Does anyone know where I can get parts for one of these barrels? I've got a M1 barrel and someone replace the front sight blade and the rear sight and I would like to put it back to the original set up. I've already tried E.R. Shaw and they said that they didn't have any parts still in stock.