The "around the corner" gun used an over-diameter barrel bent 90 degrees so it could "bounce" bullets around the corner. I believe there was even a U.S.-made "greasegun"SMG barrrel made like that either towards the end of WWII or shortly thereafter. Maybe we copied the German idea?? These barrels were not all fully enclosed-some had just half the barrel of solid steel to bounce the bullet off of.....
I do know that recoilless rifles such as our 57mm, 75mm, and 106mm weapons were developed at pretty much the same time by us, the Germans, and Soviets... parallel development.