The 6mm American is very similar to the 240 Coyote #2 cooked up by Harold Broughton back in the '80's.
That case's capacity is very close to a 6BR, except it is smaller in OD and longer.
I put a Coyote together in 1984 when the big PPC brass shortage was on. I have had sporadic periods of greatness with it, but usually prolonged periods of mediocrity. So much for trying to beat the PPC.
FWIW
Steve Kostanich