There is also another "fix" for the 870 for jams caused by short stroking.
You mark a line with a magic marker on the center line of the lifter. Drill a 3/16 hole about 3/4 inch from the front end and another about 1 inch from the rear end. Take a Dremel tool with a cutting blade, and make a slot between the two holes.
If the gun jams due to a shell in the lifter and one in the breech, you insert a knife into the slot, and push the loaded shell back into the magazine. You do not have to disassemble everything to get the gun back working fast.
This is more a fix for a "tactical" shotgun rather than a sporting one, as one might need to get the gun back in operation quickly.
As suggested, pump the thing with authority, every time!