"立式双筒短枪" can be translated as "vertical double-barreled short gun" or "upright double-barreled shotgun" (if it specifically refers to a shotgun). The choice between "gun" and "shotgun" depends on the exact type of firearm being described. If it's a general term not specifying a particular type of ammunition or use, "vertical double-barreled short gun" would be more inclusive. If it's clearly a shotgun, then "upright double-barreled shotgun" would be more accurate.