son has a 12 ga 870 rem express magnum. not new would be putting it nicely. won't rack back after firing and you have to REALLY try to get it open. sometimes ejects sometimes not. been going for 2 years now he says. chamber is pitted so i wrapped some 1000 grit sanding belt around a dowel, put it in a drill with some oil and had at it. then a 20 ga mop with flitz and shined it up. still has same problem. he shot a low brass #9 shot to test it today and racked it so hard the brass came off the shell. i took the buttstock off, sprayed the nasty greasy crap outta the innards, still won't rack UNLESS i reach in behind the bolt with a screw driver or punch and manually push the hammer back. then it racks open like greased goose shit. wadda matter?

Remove trigger group and check for loose or missing parts. Check the interceptor latch on the left inside of the receiver to see if it is broken. While you have it apart re-insert the forend assy w/o bolt. Cycle it and watch for proper action on shell latches. Inspect action bar cams for burrs or wear. Check chamber mouth for metal moved into the rim recess. Also check barrel extension for drag marks from the bolt body. Pics would be good if you can send them.

I would guess the action bar lock (right in front of the trigger guard inside the action; you can see it move out of the way when you pull the trigger or push the slide release button if you push up on the carrier like you're loading a shell) is burred or bent. Or maybe the action bar is burred or bent.

i can TEXT pics but putting pics on this site is a no go.

is the interceptor latch the same thing as the action bar lock? i can't find anything labeled as interceptor latch on 3 diff schematics i have printed. the action bar latch does show signs of abnormal wear as in shiny wear marks. however, when i manually push on it when trying to rack gun nothing happens. only when i push hammer real far back does the action slide to the rear.