Thompson submachine gun
The US military began using the weapon in 1938 and used it throughout World War II. There were two variants used. The model M1928A1 could use either stick or drum magazines, it had a muzzle brake (to counter the gun's tendency to rise when fired) and had Cooling fins (to counter the barrel of the gun getting too hot). The M1A1 model had no cooling fins, could only use the stick magazines and had a simplified sight. It was used by all the allies during the war. The main problem of the Thompson submachine gun was it was complex and expensive to manufacture. It was also heavy at about  loaded. It had to be kept very clean or it would misfire. But soldiers liked the weapon. It was a favorite of British and Australian soldiers. In the US military it was carried by squad leaders and officers. In the heavy jungles in the Pacific War, it sounded dangerously like the Japanese light machine guns causing some confusion on both sides. When the US developed the M3 submachine gun, also called the "grease gun", it was much lighter, cheaper to make and had a slower rate of fire. Most soldiers who had Thompsons were very slow to give them up in favor of the less well-liked grease gun.