This morning I saw an episode of Mail Call on The History Channel that I've never seen before. (once again my wife refused to believe there were ANY episodes of that show I'd not seen.) In it he described a weapons system from the First World War I'd never heard of, the Pedersen Device. This device was an addition to the M-1903 Springfield bolt action rifle which was the main rifle of the United States forces in WWI. In order to make the bolt action rifle more effective they modified by removing the bolt and inserting the Petersen into it. This added a magazine that held a number of special ammo rounds, the same caliber but shorter, looking a bit like the rounds for a "assault rifle." The resulting gun could fire semi-automatic.

It worked fine but was too late for the war. The device was so effective that it was labeled Top Secret until after the war.

Pederson Device