[AcceptableUserName]

same goes for a remington 870 express. the back of the bolt is very rough/unpolished. seems to me itd be smoother if lightly polished. is this recommend? and what materials should i use?

[dak0ta]

I'd recommend just racking the slide and letting it wear itself in. It's brand new so it's expected to be rougher and not as smooth.

[Dirtpile]

Plus, that Mossberg bolt is made of ultra-hard tool steel, it'll take a lot of work.
On the other hand dak0ta's advice is right on the money. Mossys loosen up nicely from just a hour or two of slide racking thanks to the aluminum receiver.
Added note: It'll go faster if the only lube on the action is just a little oil, but don't do it dry or you might gall the receiver.
One more thing: Make sure the barrel isn't on too tight. The bolt at the end of the mag tube should only be finger tight. If you tighten it too muck the action will bind.
If you do decide to polish it you will need 400, 600 (and for a fine polish 1000+ grit) sandpaper and some light oil like cutting fluid or honing oil.
start with the 400 (dry) to smooth it then switch to 600 (dry) then the 600 wet. (fine polish 1000+ dry then wet finishing up with very light pressure)

[jlv08]

I've used molybdenum disulfide paste on sliding parts of some new shotguns and just worked the slide.

As you work the slide the moly will aid in the parts working and smoothing into each parts mating surfaces.

You'll want to work your slide continuosly for it to smooth up.

It will take some time but it will get there.