I trained and drilled with the M14, a fun and accurate 7.62 at the range.
I have an M21 clone.If I can see it,I can hit it.
People who know more than me seem to prefer the 6mm Remington or .243 Winchester for longer ranges. Still I love the 120 year old 6.5X55. In modern strong rifles with the common 1:8 twist, the old timer should be hard to beat. That is with hot loads.
In the old Swedish Mausers it is a pussycat.
I think all of them beat the 5.56 or 7.62 for must uses.