I just bought a brand new Rem 870 express 12 gauge and the action is locking up after firing a round. I have to use all my strength to get the action to snap loose and eject the round.
This happens about every 3-5 rounds.
I'm using Federal 100 pack 2-3/4 #7.5 shot.
Do you think it's the ammo, or something with the shotgun? would you return the shotgun for warrenty work? I'll have to try another brand of ammo to see if the same thing happens.
I've only put 25 rounds through it so far.

I had a similar thing happen to me this fall on my 25 year old Wingmaster but assumed it was some hot 3" loads as it has never happened before.

Sorry I don't have more for you but this just happened on the last hunt of the year and I have not shot it since.

Get a 10ga bronze bore brush and some polishing compount. Put the brush in an electic drill and cover it with polishing compound. Then run the drill on a slow speed, slowly in and out of the chamber. You will see it polish up. Do not over-do it.

My experience has been that Remington does not polish the chambers as well as they should (to reduce costs) on the 870. Once done, it should take care of your problem.

tryed a bronze 12 gauge brush with some jb bore paste, then did some faster spinning with a 12 qauge cotton brush with jb bore compound. it seems smoother even though I can see the scratch marks from the brush.
I tryed a 3" buckshot mag load and it cycled smooth after firing. I'll have to try some of that federal 2-3/4 birdshot again to see if it made a difference.

The end cap might be too tight. If you used something to tighten the endcap up, that might be the problem.

I used my hand to tighten the end cap and turned it all the way till it stopped and let it fall back 1 click

Bought the same ammo at Walmart two years ago same gun same problem.Ensure chamber is clean change ammo.Mine has absolutely no problem ejecting 3.5"Black Clouds they recoil hard enough to open the action themselves.