Picnic box hole

Which shotgun should I buy? - how to change barrel remington 870

I put in the output of 870 Remington, but I would like to do two things, and has only one weapon. I want a weapon for the protection of the homeland, and I want to go to the beach and move to a position on the plate. I chatted with a guy from the local armory and told me to buy only two weapons, but do not want to spend too much money. I know I can change barrels and only a shotgun, but I'm not sure how much it would cost. I want a 18 "barrel home, and about 28" barrel. Moreover, this is my first shot, but I am very familiar with them.