US Chemical Warfare Service developed special recoilless mortars during the world war II. This first weapon was tripod mounted 4.2-inch mortar.
http://www.mortarsinminiature.com/images/M4%204.2-RM-5.jpg
http://www.mortarsinminiature.com/images/M4%204.2-RM-1.jpg
Designer of this weapon seems to have been Dr. Clarence Hickman who also worked with Robert Goddard. I have also seen a reference to 60mm bipod mounted version.
"Recoilless weapons had been investigated by the American Army in WWI. In WWII the idea was revived by the Germans, the British and then by the Ordnance Department. When the CWS began development of a horizontal mortar, it realized that the principle involved in a recoilless gun might be applied and, in October 1943, Gen. Kabrich asked Dr. C. N. Hickman, chief of section H, division 3, NDRC, to undertake the development of a mortar having no recoil. That same month, the first recoilless attachment to fit on the breech of the standard mortar was designed and, in November, firing trials were started.
Step by step, the ignition system, firing mechanism, reaction chamber and shell were perfected. The shell was fired in the usual manner. A small rocket, called a rocket driver by the designers, was attached to the fuze. When the mortar was fired, the rocket driver hurled the shell back into the barrel where it struck the firing pin. The firing pin then detonated the propellant charge and started the shell forward. The rocket driver fell off while the shell was in the air, exposing the fuze."
http://www.4point2.org/mortar42.htm
But the most interesting weapon is described in this patent:
http://www.google.com/patents?vid=USPAT2515180
It seems to describe a shoulder fired recoilless infantry rifle and the designer is Maurice E. Barker of United States Army. I did some detective work concerning this person and i found this PDF:
http://www.cheg.uark.edu/academy/documents/BarkerMauriceEugene.pdf
Does anyone have any information about this weapon? Did it ever even evolve into a real thing?