Regarding the Sterling 9mm Sub Machine Gun, I was a British Army Ammo Tech during the 1960's and carried this weapon for two years during the Malaysian/ Indonesian Confrontation in Borneo that UK and Commonwealth Allies were part of from about '63 - '66. There were concerns about the stopping power of the relatively low velocity 9mm round. The weapon was sturdy and could take a lot of knocking around during my travel around the area. It was also good with the tropical, humid and dusty environment. I still have my improvised magazine pouch at home somewhere.
During the '50s we had a cadet force at school which pupils could join from 13 years old. (I joined in '55 and left in '58 to join the Army at 16 as an apprentice.) The weapons in the armoury were all WW2 vintage, 303 inch rifles, 9mm Sten guns and .303 inch Bren LMG's. The Stens were very basic but stood up to rough treatment and worked. They were cheap and easy to produce.
Regarding shotguns, the pointman in a UK jungle patrol often carried an automatic shotgun to provide instant suppression with a barrage of pellets if surprised in the ulu [jungle]. I suppose a bit like a mobile M18 Claymore!. I cannot recall now what model of shotgun was used