Mossberg 500A hard to rack with new barrel

I have a Mossberg 500A 12ga that came with a rifled & vented barrel. Since I use it mostly for home defense I purchased a used 20" smooth bore barrel with rifle sights. When I install the new barrel, it takes a lot more effort to slide the forearm fully forward. The forearm moves freely but there is resistance the last ½” or so. I've compared the ends of the two barrels and they look identical.
Any suggestions on what is causing this and how to fix?

Greatly appreciate any help!
ddudeck

make sure the smooth's notches and od at the chamber of the barrel are identical...micrometer identical

try taking it apart and putting it back together. Maybe something didnt seat right. Is it screwed as far down as possible?