Rem. 870 14.5", 18.5", and 20" with bead or rifle sights (USA, USN, USMC) 5-8 shot parked and blued
Ithaca M&P 37 (USA, USAF, USN) 16.5", 18.5" and 20" bead and rifled sights 5 and 8 shot
Winchester Model 1200 (USA and USAF) 18.5" bbl parked 6 shot
Mossberg 500 (USMC & USAF) & 590 (USMC/USN) 6-9 shot
These are the most commom types you'll see. Older guns include various Stevens Models (520, 620, 66, 77 etc...) as well as Older Winchester and Rem Pumps and Autos.
Standard is supposed to be the Mossberg 590 and Rem. 870 but older shotguns are still in inventory. New Benelli M4 is supposed to replace them all but I doubt it.(Were still using A1's)
Only Mil loads I've seen are Fed. or Win. 00 and #4 high brass plastic 2.75". Flechettes were played with during Vietnam but were XM muntions and were all experimental. I've never seen slugs. Doesn't mean there in the system, just haven't seen them.
As most have stated they are limited issue weapons usually for MP/SP or POW details.