RemingtonOwners.com

I bought a rifle barrel for my wife's 20 gauge the magazine cap can not screw on. Is there a smaller cap?

I'm guessing you meant "rifled" barrel, for slugs? Pics would help, and have you held your old magazine cap and new magazine cap up back-to-back to make sure they are the same size?

Yes. The old style caps were designed to engage the detent on the barrel ring. The new ones work with the stupid saw toothed plastic piece in the mag tube and have a smaller skirt.