IMO, this should be a flat -5 AP - 1 per round in the burst, up to a - 8 with 6 rounds.
A max of -8? I can do better than that with a single shot, using a Barrett 122 (at -6) with APDS (-4), for a total of -10. Why would a special move that is designed to only work with the ammo I'm using, and that uses multiple bullets, give me less than what I can achieve with a single shot?
It should be a max of -8 from the action that requires you to fire a burst with APDS ammo added to the modifiers for APDS and your base weapon.
So, use this (BF) with the SMG example and you get 0 -8 -5 = -13 instead of -5.
With a Savalette Guardian, -1 -8 -5 = -14 instead of -8/-9*.
With a Yamaha Raiden, -2 -8 -5 = -15 instead of -11/-12*.
With an Onotari JP-K50, -3 -8 -5 = -16 instead of -14/-17*.
*see my earlier post about the wording of the effect and how to calculate the final modifier.
Alternatively, you can remove the ammo and fire mode requirement and change the basic called shot to +2 DV, +1 DV and -3 AP, or -6 AP.