The MP-18,I was most likely the first true submachine gun to see combat use, issued in the final months of World War One to German Sturmtruppen. These guns were originally fitted with 32-round drum magazines form the Artillery Luger, but they were almost all quickly changed to standard System Schmeisser box magazines shortly after the war ended. This is a rare example of one still in its original configuration, as brought home by a US soldier as a war trophy.
The MP-18,I set the standard for basically all future SMGs. It has remarkably gentle to shoot, with a low rate of fire and nice big sights. The drum does throw the balance substantially off to the left side, but it has few other serious problems.