6mmx222 or 25x222. Pretty slim appetite for powder.
If you make that a 6mmx223 or 25x223 it meets the OP's criteria of "reasonably available components".
My 6x47 (6x222Mag) with 85gr Partitions does a great job, (grand kids love it!) but it I were building it today, I'd go with the 6X45 due to the availability of brass.
