gnu, those were called "trounds" and they were used in a prototype pistol in the early 1960s. They were, if I remember, fired electrically and the entire project kinda petered out due to lack of interest and limited use possibilities.

Yes, there was a pistol called the "Gyrojet." It fired (electrically) .45 caliber (about 11mm) "rocket propelled" bullets. That is, the fuel continued to burn and accelerate the bullet down the (vented) barrel instead of a sudden burst of hot gases propelling the projectile, which loses velocity as soon as it leaved the barrel. Again, the use for such a weapon was (and is) limited. My youngest brother carried one when flying over Vietnam, as the tracer round he would fire was about the only thing that would penetrate the triple canopy jungle and alert SAR to his position so he could get outa there.

Want plans for a Sten gun, capable of being made in any fairly well equipped home workshop? See "The Anarchist's Cookbook," still available in bookstores and online.