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Brian one of the main things with subsonics is you want to get away from the possibility of having a secondary detonation which becomes more of a possiblity (so said) when using less than a 50% case charge. Also it is said the burn inside the case is not consistant when using a low charge leading to inaccuracy. With 6mm bullets you will probably be under 10gr of powder and that is a definite under charge in a 223 case. That is also one of the reasons for using 221 brass in addition to bullets length, the 221 is the right case capacity for subsonics. It should not be a problem to push the shoulder back on a 223, trim it and then expand to 6mm and seat a bullet. You can either make a dummy round up and have a reamer ground to its specs or find someone who has already done it. Have fun.

After posting I did some checking on reamers and dies and you may want to get info on the 6mm Mach IV. It is a 6mm based on the 221 case and both reamers and dies are available from standard sources.