I'll bet if you take a magnet to those light brass cheap shells, you'll find out they are steel base hulls; and if your 870s are Expresses and not Wingmaster, they are known for rough chambers allowoing steel hulls to expand and stick. The solution is to wrap some 0000 steel wool around a dowel in a cordless drill and use a little oil to smooth the chamber
Yessir, express magnum police trade ins, I'll give that a shot, thanks!