This is a Model 1923 Thompson Autoloading Rifle, one of a batch of 20 made by Colt for US military testing in 1924. The system is designed on the same basic Blish principle as the Thompsons submachine gun; the idea that two sliding surfaces will lock solidly together under enough pressure, and not begin to slide until the pressure drops below a certain level. In reality, both guns are simply delayed blowback, and the rifle (in .30-06) suffered from very high extraction pressures. So high, that ejected cases were reportedly sticking in a wood board at the 1924 trial.
Versions of the Thompson rifle would continue to receive military testing until 1929, and one broke a bolt in an endurance test and was pulled out for the final time. This particular rifle is a bit of an interesting anomaly in that it has a lightweight rifle barrel and a detachable magazine. In theory, the Model 1923 included a rifle version with a fixed internal magazine and a light machine gun versions with this rifle’s type of detachable magazine but also a heavy barrel and a folding bipod.