"In order to aim up or down I had to turn a knob. Is this typical of machine guns?"
Hi, Jrob. It depends on which type of machinegun you're talking about, and which sort of mount it's on; most tripod-mounted MGs have an elevating mechanism like what you describe, but these are mainly used when the MG is set up in a fortified emplacement, and the terrain is forcing an attacker to cross into a field of fire in front of you. "Flexible" mounts (where you can move the MG around to fire at will) are a lot of fun, but they can also allow the muzzle to point outside the confines of a backstop; that's probably why the MG you shot was "locked down".