870 won't cock with slug barrel on?

[cjben]

was going to shoot a few rounds today to make sure my slug barrel is still sighted in. I took the shotgun barrel off and put my slug barrel on. The gun will not cock and the slide doesn't lock. I put the buckshot barrel back on,it works like it should. I even went and found my old smoothbore slug barrel that I hadn't used it 10 years,it worked also. I just used this barrel last year without problems. If I loosen the cap a little,it will work sometimes but I really don't trust shooting it like that. Thanks for any help!!

[Synchronizor]

Check the recess in the barrel extension for wear or foreign debris. If something's preventing the locking block in the bolt assembly from fully seating in that recess, it could cause the exact behavior you're describing.

[cjben]

Sorry just saw this reply. I can't find any kind of debris anywhere. I even took a piece of 220 grit sandpaper and sanded very lightly on the edges where the barrel and receiver meet. Nothing changed. Can't feel a burr anywhere. Why I think it is the barrel is because out of the 3 barrels I have,this one is the only one that does this. I am really at a loss of what to do next besides take it to a gunsmith or buy a new barrel. Not really looking to spend a bunch of money on a deer gun this year. Guess I can put the smoothbore barrel with open sights and hope they come in close. lol The thing that has me stumped is that it worked great last year.

[Scorpion8]

The thing that has me stumped is that it worked great last year.

Well there's your clue. I'd remove the barrel, clean the barrel/receiver mating surfaces thoroughly, lube the barrel stub lightly, and slowly slide them together. I had a similar issue once when I was trying to insert the slug barrel and the receiver slide was at full-open cock. Seems like the slide action has to be "in-the-middle" of fully open or fully closed. Then the slug barrel stub mates up perfectly. And make sure the retaining ring on the magazine tube is fully engaged. If it worked before, then it's something minor that is keeping it from working today.