mods
Shotguns, especially the 870, are pretty modular and the accessory market is broad. I try and keep it to a minimum but not everybody thinks that way.
(same with M4 carbines too!!)
A mag extension for a fighting gun is not a bad idea, provided reliability is not sacrificed. We have one 870 at work that has been a real trouble maker in this regard, a combination of incorrect installation, and operator ignorance. I like the idea of 6/7+1 as opposed to 4+1 in a gun (any gun) that my life might depend on. You wouldn't select a 4+1 hand gun for SD would you? (snub .38 excluded). Seems like I have read that the 870 Express needs some tube modification to take an extension and you'd better check some more on that.
I don't care for the Sidesaddle ammo carrier. It installs on the receiver right where I want to put my hand for informal or "trail" carry. Plus, I have seen the screws that attatch them to tight and bind the trigger group, or strip out
and allow the plate to become loose. Finally, it widens the gun at an odd point that sometimes conflicts with vehicle racks.
I took the plastic clips from a stripped sidesaddle and screwed it onto a synthetic stock and it has made for a very durable and secure ammo butt cuff.
As far as stocks go, all my shotguns have conventional stocks. Reason is consistency. My hunting shotguns guns, and my issue LE/SD gun all feel and point the same. I experimented with a pistol gripped/full stocked 870 for a while. It did not carry well slung (as a turkey gun) and did not swing well for moving shots on game. It did seem to absorb recoil better, the hand placement "catches" the recoil, instead of running through the grasping fingers. And it seemed to be easier to hold "on target" for extended periods.
But the awkwardness and hard to carry features over weighed it for me.
The tiny factory bead needs an upgrade to something more visible, but durable. I have broken a lot of fiber optic bow sights, and I tend to baby my bow. A shotgun is like a hammer and will see a lot of use and hard use, the sights must be durable. Upgraded rifle sights Like Mepro's or Trijicon's are a good start, and the peep/post tactical rigs may be best.
I don't do entries, searches, or rope work or other tactical stuff, so a regular fore and aft sling works for me. Same with a light.