A long while ago a local gunshop had a racks full of Toledo Police Department M870's. The majority of them were beat to heck. Ones that had only a little finish wear got new wood. I got one of those, paid probably about $250.00
Found out why my M870 had only a little finish wear, it was because the second round jammed on the lifter!. Not the sort of weapon to take into a fight where you needed two shots.
I disassembled the shotgun and found a deep burr on the lifter. The rim of the shell coming out of the magazine tube would catch in the burr.
Took a dremel tool and contoured and smoothed the burr, and the shotgun feeds like a champ.
I find it surprising that the average Cop is not interested in their firearms. A shooting bud, a Highway Patrolman, and a supervisor, has to brow beat the guys in his company to keep their firearms maintained. He told me of shotguns stored in the back of Patrol Cars where the mechanisms were frozen from rust. Took a bit of banging to get them to function.