Tough call, since shotguns tend to turn up through irregular channels in Hot areas.
Our folks use 870s, 590s, 500s, 37s and some Benellis. Canadian troops have some of these also.
Other shotguns in service include the A-5, more Benellis, and some off brands in Africa we never had.
Betcha variants of the Kalishnikov like the Saigas are out there also.