If it is an express take it to a gunsmith. On the express it is pressed into the rib and there will be no threads when it is pulled out. It will need to be tapped for the new sight. If it is a wingmaster it may screw out, I just don't remember.
Just had this same problem yesterday,
I have a new 870 express, and my friends that helped me with it, were thinking it screwed out like all the others do, but it snaps in. we were all surpised.
pliars worked fine.
most later year model Remington shotgun barrels have the bead "staked" in place
this is where a hole is drilled and then the bead itself is placed into the hole and then the metal at the top of the hole is depressed to deform it which in turn holds the bead in place
to get it out is fairly easy "sometimes" but other times is a booger
one can remove the bead with a good pair of side cutters with the rib protected to keep from marring it. simply grip the bead with the cutters gently enough not to cut it but holding it firmly, then pull upwards while twisting it to get it out
if this does not work and you break the bead itself off , then i recommend a milling machine to mill it flat with the top of the rib and then using a beadsight to cover the existing hole by drilling and tapping behind this factory hole enough to utilize a long style bead which will cover the factory hole once it is done correctly
the hole is not threaded if you remove the bead so it must be tapped with the correct thread to enable a screw in type bead
a 6x48 thread size is the norm for this as the hole is the correct size for tapping this thread once you get the bead out
the 6x48 thread is a gun related size thread only and you will not find a tap this size at your local harware store, so taking it to a smith is the best route