[Brother in Arms]

WW2:
M97 Winchester Riot and Trench
M12 Winchester Riot and Trench
M10 remington
M11 Remington
M31 Remington
M30 Stevens
M520 Stevens Riot and Trench
M620A Stevens
M720 Savage
M37 Ithaca (my favorite) Riot and Trench

Vietnam they added
M870 Remington
M870 MK1 Trench (used by Navy and USMC)
M67 Savage
M77E sevens
M1200 Winchester Riot and Trench

Of course as they added newer shotguns in Vietnam but many of the older guns were still in use I have seen several photos of soldiers carrying M1897 trench guns in the RVN! I believe New M37's where purchased for use in VN (these were favored by the Navy seals and other a special forces groups)ARVN troops are often seen with 520 and 620 stevens shotguns and the M77E Stevens is said to be the most issued shotgun during VN but most where left to the ARVN at the end of the war as they where considered unsatisfactory for military service due to stock attachment point being weak often broke off the back of the receiver.

Coldwar Era:
M500 Mossberg
M590 Mossberg Trench

Where added to the in the Coldwar but some of the earlier shotguns were still being used most forgotten being the M1200. In fact I read a briefing from new Iraq war telling units equipped with old M1200 to turn them in for the new Benneli Joint tactical forces shotgun.

Best bet for twilight:
Mossberg 590 which would be the newest shotguns in the inventory and "replaced" the 870 Navy and Marine units. Though there are several pictures of Navy Seals carrying the 870 Mk1 in the 1990's. The Navy used a lot of Mossberg 500 on its ships toward the end of the 80's and my freind who was on a ship in 2000 said that is what his ship was equipped with.
Winchester 1200 for some army and air force units (saw a picture form the first gulf war showing a female soldier carrying a M1200 Trench)
Ithaca 37 could still be found kicking around in some unit inventories again navy being the most likely. But by the 80's I would suspect most of them had been replaced.

another interesting note I haven't seen modifications added to U.S. military shotguns from the coldwar era. But I have seen a picture of a commercial Sythentic Choate stock on a 870Mk1 Remington in the hands of a Marine and a factory Remington police top folding stock on a couple Remington 870Mk1 being used by Navy. I saw a picture of a top Folding stock on a Winchester 1200 Trench, and a commercial (butler creek or choate) collapsing stock on a mossberg 500. I have seen slings fitted to the Mossberg500, 590 ,870 but not as often as with none especially in older photos.

Brother in Arms