The Swedish K
The Gustav M/45 was the standard submachine gun for the Swedish Armed Forces from 1945 until the mid 1960s. Created by Carl Gustav (hence the name), it saw action on multiple fronts, one of course being Vietnam. It is a blowback operated, automatic sub machine gun with a detachable 36 round magazine. It is a cheap stamped sheet metal weapon, taking techniques from the German MP-40 and the Soviet PP-Sh. There was a perforated tube around the barrel to act as a cooling system as well as protection from the soldier's hand (the correct way to hold the weapon was one hand on the grip/trigger and one hand in front of the magazine). It had a rectangular, metal wire butt stock that was collapsable in order to be more portable. It fires the Swedish 9mm x 19 M39B Parabellum cartidge. Gustav and his company signed agreements with Indonesia and Egypt to have it built in the early/mid 1950s as well. The M/45 saw service with the Swedish Police as well, finally being retired in the mid 2000s.
As for Vietnam use, the U.S. Special Forces and Navy SEALS adored the weapon. One notable feature that the SEALS liked was its ability to fire almost immediately upon surfacing from the water. Often times when operating with the weapons, the markings were scrubbed off in fear of capture. As the United States began becoming more deeply invovled in Vietnam however, the Swedes issued an embargo upon the U.S. which ended their arms trade. The U.S. Navy began becoming interested in the M/45 at the same time, so the Navy contacted U.S. armmaker Smith & Wesson to produce their own copy, the S&W Model 76. By the time it was prepared however, the Navy had lost most interest in it. The NVA and VC did capture some copies of the M/45 as there are pictures of them holding it.