The background on this is that this 870P belonged to my job and was a range gun. It would give guys problems with locking up and being near impossible to work the action after firing. It only did this occasionally and since we try our best to "train like we fight" and all, when it happened no thorough examination of the failure was usually done at that time, just the instructor doing whatever it took to get the empty shell out. For the most part, for better or worse, when something goes wrong with equipment the assumption is that its the shooter until it happens to a known good shooter and a lot of the time these things are happening on a range, in the dark with four other guys on the line trying to qualify.

Long story short, eventually it happened to a known good shooter and they found a broken ejector (ejector rail, I believe, definitely not just the spring). The shotgun went back to Remington and the ejector was replaced but it seems like that was just a symptom of the real problem. In talking to one of the firearms instructors he told me that they were still having a problem with the gun locking up after it had been fired and that since it was about time to replace the range shotguns anyway he would be taking it out of service. When we take guns out of service they are sold to a distributor and when I heard what the distributor was paying, for all my shit talking about 870s not being that great based largely on seeing problems with this very gun, I couldn't help but make an offer of my own. So now I have it, and I'm working on making it the gun it should be. Why wouldn't I want to take on this headache, right?

Range time around here is limited and expensive so I'm trying to address any obvious issues before I take it out since I know that there is a problem.

Since the locking up problem only ever occurs after it's been fired and never with dummy rounds or unspent shells, I was thinking it might be the common rough chamber. In searching around this definitely seems to be more of an Express problem and this is a P. Also, I felt the chamber as much as I could with my fingers and it actually felt pretty smooth, even compared to my Wingmaster that is at least 40 years old. Further, the only thing this shotgun has ever been fed are Remington Managed Recoil Slugs and Buckshot and, briefly Federal Flight Control Buckshot. ("Train as you fight" and all) None of the poorly regarded cheap Winchester rounds. Never the less, since there was really nothing to lose I did the bore brush/steel wool polishing job on it and now it feels really smooth.

While doing that and checking over other parts I discovered what seems like an odd condition on the locking block and now I'm wondering if this could be the culprit. It is kind of flattened on the back edge of the part that locks into the barrel extension. Since it's part of the locking mechanism, I also want to confirm that the gun is safe before I try to find out if my chamber polish worked. Does this look okay?





So, 870 experts, could this be the issue?

The locking block is chipped to me but probably not THE problem, but A problem.

What remington told me years ago, the problem you are having is that the "brass" on cheap shells is actually steel and doesn't contract to its original size after firing.  Use a magnet on a spent shell and you can tell for sure.  Use some brass shells and see if the problem continues.  There might be a burr in the back of the chamber.  If so, polish it out and try it again.

Thanks. How big a deal is it to replace the locking block and do you think it's worth it?

I would be surprised if the issue was shells. As I mentioned in the post, the only thing it's ever been fed for its whole life, aside from a very brief stint with Federal Tactical was Remington shells.

A punch and hammer is all you need to replace the locking block.  
Try some different ammo to see if that helps.  Also, use a magnet on the brass to see if it really is brass or brass plated steel.
Since you said it works fine with dummy rounds or live rounds, but sticks with spent shells, I suspect the chamber is rough.

Thanks, that's where I was leaning as well. I did the steel wool polish and its quite smooth now, so I guess I'll run it soon and see how it goes.

Check the locking lug recess in the barrel extension to see if it has any burrs that the locking lug could hang up on. Lightly file them away if it does, this was a cause of locking up on my buddy's Express and once removed we greased the lug and worked the action till it smoothed out.

A brake cylinder hone may be needed on the chamber before a flexihone.  They can be pretty rough.  You can also hit the cone in front of the chamber really hard and get rid of some of it for less recoil and perhaps a better pattern.  

I have done this several times, once to a rusty chamber, and to a couple double barrels, and it would take some real effort to remove too much material.  

I finished with fine sandpaper and emery cloth wrapped on a stick with wadding in my cordless drill.

I was going to say this. one step further would be to change barrels with a gun with no problems. If it does it with another barrel then it is most likely the locking lug is out of spec, likely oversized, and the chip in it is a symptom of that. If the barrel from that gun causes it in another gun then it is in the locking lug recess and some light filing and shaping is in order. This is assuming that the gun is clean and that there is no accumulated rust or crud or forearm interference that is preventing the barrel from seating properly in the receiver.
Pics of the locking lug recess and barrel to receiver fit and barrel ring to forearm area will help.

Thanks everyone. Would the locking lug issue only effect the shotgun after it was fired? It has never hung up in the least bit unless it was fired.

It could. The shell / shell rim expand upon firing and keep the bolt pushed back a little tighter after firing.

Thanks for the replies. I checked the barrel and there are some very slightly raised areas around the recess though none seem to line up with the chip from the locking block and none on the inside of the recess.
More of the locking block:



Barrel Recess:





Inside of the receiver:

The wear on the forward edges (towards the chamber) of the barrel locking recess looks as though the locking block is striking it and peening the edges slightly. I wonder if the recess itself is out of spec? Whoever suggested a barrel change and function check, may be onto something. Might be cut too shallow and thus too tight?