Direct feed breech/body. CCI sells them as left, center, or right feed. Other than that, if you've got a 12-gram changer as the rear part of the body, you need the rear ASA part (also available through CCI). If you run a 12-gram changer as an add-on either vertical, off the rear ASA, or on a bottom line, just replace the changer with a tank.
Phantom's are pretty modular. Wish I could get comfortable with one.