What standard caliber would you get? (.220, .223, .243, .308, .25-06, .22-250)

I want a round that is small and will push a piece of lead to 400 yards without any travel which I know is impossible but?

Say I' m 3" high at 100 and 4 inches high at 300 and dead on at 400 is there anything this flat?

perhaps a .220 swift I know barrel wash is a factor with this gun. What would you choose.

I have a friend with a 7 STW that is attractive but don' t want that gun.

Jason