[amafrank]

I guess the basic answer is that subsonic means its not armor piercing anymore. Once you drop the speed than you lose the capability you are looking for. If you want subsonics you'll have to deal with the lack of high penetration. The hardness of the projectile has little to do with penetration, its much more related to the speed. You can see this by shooting lead cored copper jacketed projos at high velocity against a steel plate, then try the same with a hard cored penetrator at low speed. We've tried this by decreasing the velocity of the projo in steps to find where the penetration drops off. Once you drop below about 1800fps the penetrator seems to help very little. If you could get the weight up a bit the values improve a lot but with the 62gr slug you won't do much better than the lead Aquila bullet for penetration.

In any case, if you are determined to go the subsonic route with the .223 than yes, you need to find a powder that will fill the case as much as possible to insure even and consistant ignition and burning. Not sure which to suggest as I've not dealt with cartridges using such light projos and so much capacity. A faster burning powder might do the job though guys I know who've tried trailboss haven't had much luck. I have heard of others trying to reload the 5.7 FN cases with heavier bullets and not having good results. The word is that the 5.7 is pretty tricky to reload well. Its worth a try though....

Good luck in your search

Frank