870 Express with Wingmaster Barrel
 

Can I change the barrel on my 870 express to a wingmaster barrel ? I have a opportunity to get a used wingmaster barrel and wanna see if that's a good Choice .

yes.....it will be fine

What are the differences?

only the finish

Well, make sure it is chambered for 3", because a lot of early Wingmasters were not. It will be lighter in the same length, and the top of the rib - if it is ribbed - will have anti glare finish as opposed to the same finish as the rest of the Express. And several million of them were made before RemChokes came on the scene.

What would happen if I put a slug barrel on a regular 870 (not wingmaster, express just a Remington 870) , would it shoot slugs?

870 12 ga barrels interchange. Only the 20 ga's have interchageability issues. There are several versions of 20 ga's that use very specific barrels.

You can shoot slugs thru any Rem barrel(slug barrel or not). Most would certainly shoot better using a slug barrel. Virginian is correct about some of the early 870 barrels being 2-3/4" only. Rumor has it that at some point Remington changed to chambering all 870 barrels to 3" but marked some 2-3/4" when sold with a non-magnum gun. I never bothered to verify this rumor.
Also Rem did make the Special Field in 12 gauge which took a special barrel.

Be advised that most of the "just regular 870s" that do not say Wingmaster and have a flat finish are Expresses without the name rollmarked on them.