Reloading/Recycling Lake City M855 to Subsonic ss109 Q

[Kusarigama]

I am well versed in Aguila's cartridges and champion Super Colibri. The reason I want to reload ss109 is to have a subsonic round with armor-piercing capablility for the occasion when such a need may arrive, and a .223 is at hand.

Is the consensus to use a higher volume of powder to make sure the case is full?

Might I get better results if I reload ss109 in a 5.7x28 case due to it's lower volume and just get the appropriate upper?