The T31 is a prototype bullpup automatic rifle designed by John Garand.
A light rifle program was initiated in 1945, where a new infantry rifle was to be developed using the new T65 cartridge, which would be developed into the 7.62x51mm NATO bullet. Intially, the project only involved Earle Harvey's T25, but John Garand and Cyril Moore joined the project. The T31 was the most unorthodox of the 3 designs submitted. However, Garand retired from the Springfield Armory a while later and the design was never completed.
The rifle was the last design John Garand would work on. The operating system he developed for the rifle harkened back to the gas trap idea used on the early production M1 rifles. The entire barrel was surrounded by a sealed cylinder, and the muzzle was located slightly back inside this cylinder. When fired, some of the expanding gas at the muzzle would bounce back into the cylinder, creating a shock wave traveling down its length. At the back end of the cylinder was a short-stroke tappet type piston which would transmit the force of the gas into the bolt, cycling the action. This system was intended to do several beneficial things. First, it would function based on the exit pressure at the muzzle, and not depend on having a particular pressure curve as the bullet moved down the barrel (changes in pressure curves with new ammunition caused the demise of Garand’s first primer-actuated rifle design). Second, it would absolutely delay the action from unlocking until after the bullet had left, thus ensuring safe pressure when extracting spent cases. Finally, the redirecting of much of the muzzle blast would attenuate the concussion of firing, and recoil as well to some extent. It had a similar barrel jacket to a Lewis gun, and had the rear sight of a FG 42.