"All brass shotgun shells were used in WW2 and Vietnam because they wouldn't swell and jam the gun like paper shells."
The first shotguns issued to American troops in World War I were issued with paper shells. The drawbacks became rapidly apparent, and they were quickly replaced with brass shells.
After World War I, the military largely eliminated shotguns except for some special cases, thinking that they wouldn't be used again, so when American troops were issued shotguns, it was again with paper cased shells. These were an absolute disaster in the Pacific, and Western quickly started loading brass shells again, the M19round.
By the time Vietnam came around, plastic shot shells were increasingly in use with the military, and paper cased shells were only being used state side for training.
Early on WW II era brass cased shells were issued, but these were quickly depleted and replaced with the XM162 plastic-cased shells with the classic 00 buck load, usually civilian production red hulls with the commercial manufacturer's headstamp, but military markings on the side of the hull.
Later the XM257 round was issued, which swapped out the 9 00 buck pellets for a load of around 27 pellets of No. 4 buck.
The other Vietnam-era round was the ill-fated flechette round. These proved to be very unpopular as they weren't very effective, and they were rather quickly withdrawn.
In the 1950s the military started issuing multi-purpose survival rifle/shotgun combos to aircrews. The shotshells, developed and supplied by Western, were all aluminum cased and generally loaded with No. 6 shot.
They were not intended to be used as combat weapons, only for survival purposes in hunting food.