The business with the machine gun on the half-track?

I don't know much about automatic weapons but I was wondering what is the thing about Hartman and the .30 caliber machine gun on the half-track? The enlisted man loads the ammo in the box. Hartman then racks the cocking handle, aims the gun at the man, and pulls the trigger. It doesn't fire. He then swings the gun to the side, cocks it again, and it fires. Hartman then threatens the other man saying along the lines of "do that again and you're in trouble." What was he getting at? Not to load a weapon and leave it unattended? Why did the gun not fire the first time, but did fire the second time? What's the deal with whatever the enlisted man did with the .30 that was so wrong?