You have two choices when using Precision AC ammo.
1) Make a standard weapon attack using your full S/M/L damage values (not just the AC values). You do not get a to-hit bonus when doing this, but get a 1 point damage bonus versus "fast" targets.
2) If you really want the to-hit bonus, just use the AC special ability. You get the to-hit bonus, but only use the AC's S/M/L damage bonus.