When I was in high school in Salt Lake City from which I graduated in 1954 our school ROTC had all sorts of military guns that we were trained to disassemble. Among others, we had BAR's and both M1917 (water cooled) and M1919 (air-cooled) machine guns. We only tore down the M1919 and I remember that the main bolt spring was a real beast. It was retained by a slotted screw which we'd push down on a wooden work bench and give it a half-turn so the bench would catch the spring and nobody would be injured or blinded. We also had a rifle range in the basement of the school with a full complement of ,22 target rifles (Winchester Model 52).