rantingrelentlessly:
A Rem 1100 makes a fine SD/HD shotgun. The 1100 12ga I used to take my first pheasants is on the other side of my keypad-access locked closet, ready to roll. I shot it enough that it is as natural as riding a bike to point it where I want it to go. It may not be up to hard 3-gun competition use, but that was not the question. And not the use to which I put my 1100. Keep it clean and replace the O-ring every few years or so if you are using it lots.
To make your 1100 a better HD/SD all you _really_ need is an 18" bbl. You may or may not need to open up the gas vent holes. Before you do that, pattern and find a good buckshot and good foster slug load with that 18" bbl. Yours may be dead-reliable with your best-patterning load, no modification needed. If so, don't muck with it. Used 12ga 1100 bbls can be found reasonably and cut to 18" & a smidge. Put on a bead if you like. Or buy it already 18".
If you want to do more, I would suggest the following:
* Weapon light. Can attach one of several ways.
* Mag extension (+2) & spring, along with bbl/mag clamp.
* Sling & swivels
Here is mine after 18" bbl & mag extension, before light, sling, and mag/bbl clamp:
Some patterned loads:
Image of cheap Rem 12ga Express 000B at 7 yards:
Rem 12ga Express 000B at 15 yards:
I have subsequently patterned my Rem 1100 12ga with Federal FliteControl 00B. At 15 yards they do not so much as pattern as rip one ugly ragged hole smaller than my fist. Nice, but not necessary for HD if you want to buy cheap stuff. Just find the cheap buck that works for you. And the cheap Wally World Winchester Value-Pack slugs shoot fine, too.
FTR, I also have a couple social pump shotguns (Rem 870, Ithaca 37 DSPS). I like them all, but I shoot the Rem 1100 fastest.
If you want a cheap HD gun fast, with no build time, just buy a used police trade-in Rem 870 Wingmaster Police. Very smooth pump.
Good luck and remember to pattern your gun and practice with it.