Some do, some need fitting, ya just takes your chances, any barrel can be made to fit if appropriate for the type of frame, see the barrel fitting info in the FAQs and Help sticky. Most folks buy em, try em, if they don't fit, sell em. But I've fitted at least a dozen barrels using my barrel fitting info and probably twice that many have fit with no changes, but the advantage of having multiple frames is you'll just about always have a frame that a used barrel will fit with no changes or just minor fitting.
Read thru the FAQs, there's a multitude of things in there that you
need to know.
Tim