Any recommendations? This is a 14" cylinder bore.

I don't shoot it much, but would like to keep some first-rate defensive ammo on hand...just in case.

Best 2 3/4"?
Slugs?
3" (should I even try??)

Hornady TAP

My old dept. issued them with Fed. low recoil oo buck and low recoil slugs, the buck paterned well out to 15/20 yds ad the slugs were suprisingly accurate out to 100 yds. I'd stick with the low recoil stuff due to muzzle flip with that short barrel, 3" stuff sucks out of super 90's, very stiff recoil with slow followup shots. I've never tried the Hornady's but they do get very good reviews.