870 barrels are designed to "drop in" However, there are different contours (the "featherweights" class) that don't interchange.
Too, if you have a 2-3/4" receiver, you can't do the "drop in" with a 3" barrel & shoot 3" shells. Bad juju. You can stick a 2-3/4" barrel on a 3" receiver w/o any problems AFAIK.
Post what receiver you have here before buying another barrel - only way to be sure.
Frankenstein, my homebuilt and overpublicized for anything flying 870 is a parts gun.And I'm far from being a gunsmith. There's an old thread on it here in the Archives. It's called "Building a Kitchen Table 870" or something.
870s just go together w/o fitting. They're designed that way to reduce labor costs.