if you goto: http://www.ww2f.com/weapons-wwii/12172-shotguns-ww2.html you can read the existing discussion from a few years ago, for my own addition let me include this, since I didn't participate in this one: The M97 Winchester (Browning design, 1897) is a pump action shotgun with an exposed hammer which really started being used by the U.S. military during WW1 for trench warfare, then due to a temporary shortage of weapons many were re-issued during WW2, I’m not positive of the numbers, since I have seen different ones in different sources every thing from 26,000 to 58,000 re-issued and put to work in WW2 with some even sent to Great Britain for the Home Guard. Like the newer Ithaca M37, the older M97 has no trigger disconnect allowing the gun to fire as fast as a semi auto. The M37 is also a pump action shotgun and is one of the several types of commercial shotgun purchased by the U.S. military during WW2 and modified for military usage. The great majority of shotguns were used for training or for arming Military Police guards in US service. They were occasionally issued for combat in urban areas in the ETO or jungle fighting in the PTO. Due to different battlefield conditions they were actually more common in the Pacific theater and were very popular with the U.S. Marine Corps as you mentioned. Most of the Ithaca M37s issued for combat were modified to accept a bayonet. There were also the new Remington M31s (pump), M11s (autoloaders), Stevens M620As (pump), and more than 80,000 Winchester M12s (pump). THe M12 was a more streamlined version of the M97 with an enclosed hammer. The Remingtons, and Stevens were issued in smaller numbers for distribution to both theaters for the mentioned uses; MP guards, urban house fighting, cave clearing (PTO), and jungle fighting (PTO). I have never been able to find any Geneva convention which explictly "outlaws" a shotgun, although one of the early posts mentioned "lead shot v steel shot", that might be connected to the outlawing of "dum-dum" bullets which made the FMJ the standard for military ammo, I dunno.