The Grant Hammond .45 pistol is a gun which was too late to take part in the major 1907 US military pistol trials, and which was instead presented proactively to the military in hopes of supplementing or replacing the current issue pistol (the Remington M53 falls into this same category). Unfortunately for Mr. Hammond, his design just wasn’t good enough to pass muster (the Remington actually would have had a real shot at adoption had it been in the 1907 trials).
Hammond’s early patents were for an exceedingly complex design, which combined elements of blow-forward and long recoil operation (you can see my video on a Hammond .32 prototype), but by the time he was making the gun in .45 for trials he had simplified it considerably. As proposed to the military, it was a short-recoil mechanism with a vertically-sliding locking block. The first prototypes would actually eject their magazine automatically when it ran dry, but this feature was not like, and was removed from later production.
In total, only a handful of Hammond pistols were made. They underwent several military tests in 1917 and 1918, and proved quite accurate – but not durable or reliable enough for further consideration.