Frankly, reducing damage by 0.7
and dropping the size modifier by one (pi++ becomes pi+, etc.) is excessive for a human target. Really, what's the significant difference if the projectile passing through your body is steel or jacketed lead? (Ignoring fragmentation.) Most modern small arm AP is really a sort of APHC, anyway (see the
M993 frex) so you should probably only get less damage if the bullet has to penetrate significant DR- for instance, if the AP modifier makes the difference between a penetration or not, shaving the soft outer layer off the smaller penetrator.