Dear Faldage: A guy in guerilla warfare may have weapon preferences, but a short life expectancy if he indulges in crotchets. And gun enthusiasts love nothing better than a chance to give a spiel about their preferences. Remember Hopalong Cassidy, who favored an old heavy weapon, when the rest of the crew carried newer more rapid firing smaller caliber weapons? He was a marksman, and one of his heavy slugs on an opponent had more authority than a half dozen smaller caliber rounds. But no clues are given for the guy in the quote having a preference. And no ammo? I don't know what ammo the Stens take, but obviously 7mm Mauser ammo would be no good for them.

The Sten submachine guns take .45 ammo Here is a URL about the Mexican Mauser. Ammo is not specified.

http://www.pmendoza.com.mx/history.htm