Automatic weapons weren’t new. Every squad had an automatic rifleman, and submachine-guns were used widely. The question was whether it was worth equipping every rifleman with an automatic weapon. This wasn’t obvious at the time.

The M1 and M14 had superior range and stopping power. Being able to reliably kill the enemy and not end up outranged are intuitively important. Semi-automatic weapons also mean you don’t have your entire army burning through ammunition. When you’re fighting expeditionary wars, this is a major concern because every bullet has to get shipped halfway around the world.