Gotta cover the STG-44, it was the predecessor to the perfected AK-47 and overall first economicly mass-produced stamped-steel selective-fire-mode single-person assault rifle.
Civil war erra gattling gun and the first browning machine guns of WWI (that were refined into the browning .30 and .50 cals of WWII) should definetley be in the list too.
You can do the M60, which was based off the German WWII (infamous) MG42 and MG34 (which if you do the MG42, you gotta touch on the 34) designs.