I just bought a Remington 870 Express Super magnum, and want to change out some things like finding a shorter self defense barrel and find a pistol geip full stock. What is the difference between the regular 870, the 870 Express, and the Express Super magnum?

Do stocks and barrels fit all the models, or am I looking for something only for the Suoer Magnum?

TACTICAL

Maybe find it here;

Stocks are the same.
Most barrels will interchange, but you won't be able to shoot 3 1/2" shells in a barrel that's chambered for a 3" or shorter shell.

I don't know if anyone makes a short barrel in 3 1/2".
The Super Magnum wasn't intended to be used for defense, it was intended to put up more shot to reach high flying geese and ducks.
Recoil is extreme for defense use and actually reduces the speed of follow up shots or shots on multiple targets.

You are not alone. I, too, run an 870 Express Super Magnum purely for home defense. I got it from my brother-in-law, brand new in box/unfired for $100 (he was REALLY hurtin' for the cash). I swapped the 28" anti-aircraft barrel for an 18.5 inch with plain bead sight. I don't plan on using any 3.5 inch Goose loads for HD, anyway. I use Federal Power Shok 00 Buck. The only addition for now will be an XS big dot, sling, and possibly a magazine tube extension and shotshell holder of some variety.

If you want a shorter barrel chambered for the 3 1/2 inch shells, you'll have to cut one down. As has already been explained, they're built as hunting guns. That being the case, I sent one of my Super Mag barrels off to GunCat to have the barrel shortened and the rib removed. Other barrels will fit on the shotgun and fire up to whatever that barrel is rated for as far as shell length. The trigger pack housing and bolt are different as they're both designed to allow manipulation of the longer shells. The ejection port hole on the side of the receiver is longer as well. Not sure, but I believe the lifter is also different. Also as mentioned, 3 1/2-inch ammo may not be the best bet for defense; look at something like Federals LE 132 00 Buckshot load, great ammo for social situations.

19-inch barreled 870 Super Mag.

Nice job on the modified 870 Super magnum.

I will probably just find a 18 1/2 inch defense barrel and then I can always use the original to hunt with.

We took it out to the trap range on Sunday and right out of the box with an improved cylinder choke and value pack shells it shot 15 for my son and 21 and 22 for me. I have not shot any trap this year and only a few rounds in the last several years, so a little practice would obviously help the scores.

I discovered it does not like the Winchester value pack shells, but seemed to do fine with the Remington shells. The Winchesters would not eject from the gun and made it very hard to open the action after being shot.

Will have to try it with regular brass base shells and after it is broken in a little better. Will be looking for a fiber optic front sight, as the little factory bead is hard for my eyes to pick up on.

(UPDATE) Got a fiber optic front sight for it and picked up some Federal shells to try. Hopefully this Wednesday will have some time. Heading out to trap range Let's See One in Auburn, Illinois.