In this video, Ian McCollum of Forgotten Weapons looks at one of John Garand’s early rifles. The Model of 1924 Trials Rifle was primer-activated. That is, the primer would come out of the pocket in the brass and push a small piston back. This would serve to unlock the bolt and the autoloading process would go on from there.
This is the first that I’ve ever heard about such a system and I find it both intriguing and horrifying. Intriguing because it simplifies the barrel of the rifle – no gas ports needed – and horrifying because of the potential for failure or worse.