RonPossible t1_j1283lt wrote

First, the US exited WW2 with a few million M1 Garand rifles. In the post-war drawdown, there was no urgency to replace that proven platform. Development of the replacement began in 1944, but really didn't go anywhere until the Korean War.

The UK proposed the .280 British round, in part based on the StG-44's 7.92 performance. The US rejected anything under .30 and found the .280's ballistics substandard compared to the .30-06. The 7.62mm was selected because it's ballistics matched the .30-06 due to newly developed powder. Also because the US Bureau of Ordnance wasn't keen on a non-American design.