Primary shotgun used by the military is the Mossberg 500 in to 4 versions. Around 30-35,000 are currently in use with new contracts going out fairly often:
Mossberg 500A1 NSN 1005-01-371-4462 (USMC/Navy) 17" barrel
Mossberg 500A2 NSN 1005-01-371-5939 20" barrel
Mossberg 500 NSN 1005-01-085-1032 or (Army) 18 " barrel
Mossberg 500 MLS NSN 1005-01-295-1832 18 " barrel
The 500 and the 500A1 are the most common I believe. The 500A1 is most commonly known in the civilian sector as the 590. The 20" 590 with the heat shield and baynet lug was procured in rather small numbers also. Since these have been in procurement since the 70's with very little attention to the procurements you will find variations to include some 14" models. The Afghan Army and Police have also recieved 14" 590's.
The USMC also uses the Benelli M-4 type classified as the M-1014. Total of around 10,000 after the current contract is delivered.
The Air Force is using mostly Remington 870's I believe. The Air Force Security Police also use some of the 870 MCS SOF uses some cut down 10" 870's for breaching and also has some of the Remington 870 MCS's. Total MCS's that have been procured is arounf 3,000 after the current contract is delivered. You will also find some 870's in various units that have simply not been turned in.
The Winchester 1200 was in service, but TACOM directed that they be turned for replacement with Mossberg a few years ago. The message included the 870's, but did not list them by name.
The XM-26 will never go to procurement in my opnion. We used the early model in Afghanistan.