The M-113 was a 13-15 ton aluminum box, slab sided, and very thin. The idea was to provide an amphibious troop carrier(they do swim, but its kinda scary.) to cross the usual rivers, lakes etc.(It would swim only with the full load of infantry, or an equivalant weight of cargo, or ballast, being bow heavy) I'm not sure if the Soviet BMP had anything to do with its being adopted. It had little defensive capability, just one M-2 .50 cal at the Commanders position. The later A-Cav version had 2x .30 cals and the .50 with armor shields. Better indeed. We called them Paper Cups (P.C.) as they wouldnt stop very much incoming. RPG's would gut them, ripping large holes in the aluminum, any larger mine, or bomb would do them in as well. Even the Fletchette round darts would go through. The best places to aim on a 113, is either the left rear, (fuel tank, 80 gal. Diesel) or right front, (Engine) Wasnt my favorite AFV, but it was what we had at the time.