Slightly edited from it's orginal form:
The Combat Shotgun in the Infantry Brigade
Army Qualification Standards (Modified)
Just about every thing you want to know about Mil use of the shotgun.
Primary Service shotgun is a 18 1/2" Mossberg 500 for the Army, and a 18 1/2" 590 for the USMC, both 5+1 with bead sights. MOST non-SOF 870's in service are pushing 30 years since they were procured. SOF has a mixture of 870's 500's, 590's and the 20" 590.
The Benelli M-4/M-1014 is also in service with the USMC in fairly small numbers. I believe the Air Force is using 870's as is the Coast Guard.