I picked up 2 used in good condition 870s in 12G.  they are both express models with the 18.5 barrel and factory plus 2 extensions.  one has a stubby forend and a tac star shell carrier and the other has the sporter forend.  Both of them occasionally have issues extracting a fired round from the chamber.  It is almost like the forearm is stuck.  When it gets stuck, I have to hold the slide and slam the buttstock against the ground to get the fired shell to come out.  This only happens with fired shells.  The ammo is a mix of winchester and federal buckshot and birdshot.  It happens about once or twice every other time the mag is loaded up.  I cleaned both guns when i got them, any ideas?

Polish the chamber to clean up any roughness.  Take a bronze cleaning brush and wrap it in 0000 steel wool and use some MAAS or Flitz metal polish.  Put this in the chuck of your battery drill and clean up the chamber.  You can polish the whole bore while your at it.  Keep the "polisher moving back and forth and change direction a couple of times...Should only take a few minutes, costs pennies and will probably clear up your issues.  At the very worst, you will have a very clean shotgun barrel.  Make sure you clean the polish out with some CLP when your done.  WARNING, the polish will go everywhere when you tun the drill on, stick it in the barrel then turn it on.
Some of the cheap target loads are notorious for sticking....If this polishing does not clear up the issue you may want to talk to a shotgun expert like GUNCAT at Rose Action Sports or JD at AIP Tactical

Also, I personally think the TACSTAR side saddles are not worth putting on the gun.

I'm having this problem with my new 870 express as well.  Basically what is happening is your forend is sliding too far forward and is only being stopped by the barrel nut instead of the shell bar inside.  Take your barrel off and rack the slide several times.  When you rack it forward (Away from the action) the rails on the foreend tube should catch on the shell bar inside the receiver on the left side.  The foreend should not be able to slide off the gun.  If this is not catching your bolt and foreend will slide completely out and off the gun.  I thought I could fix the problem by bending the shell bar inside and broke it instead.  I ordered a new one and just installed it today with the same results.  Next time it locks up, reach inside the left side of the receiver and push in on the little bar along the side it should allow you to rack the gun.  I don't know what the fix is at this point.  I guess I'm going to have to send it back to see what else it needs.  Something must not be machined right which is stopping the forend bar from catching the shell bar and allowing it to slide past and "lock" forward.  I hope this does not sound too confusing for you, I'm not sure exactly how to explain it without photo's.  I did swap out my foreend tube with a friends and my tube will lock with his receiver every time so I know it's not the tube.  I'm thinking the slots for the two rails in the reciever are not machined perfect allowing them to "miss" each other and not lock as they pass each other????

EDIT

So today I took everything apart and the foreend will still slide off without pushing in on the shell latch on the left side.  The actual foreend rail is sliding behind the shell latch for some reason.  I re-assembled the gun and made sure the barrel nut was extra tight and it's not doing it any longer.  My suspicion is that I didn't have the barrel nut tightenend all the way and it was allowing the foreend to slide out too far.  I haven't shot it yet but that's my guess at this point.