[Easy]

The formula for body injuries is based on the AMOUNT OF TIME A PROJECTILE stays on target.

Soft bbs will deform on impact and this will add to the time on target.

Hard bbs will not deform and will simply rebound off the target.

Penetration depends on a lot of things and mostly VELOCITY. At equal velocities the MASS of the bb is what counts. As can be seen from the pictures.

The heavier DIGICOMs (0.42g) and the G&G (0.28g) penetrated more than the BBBMAX (0.27g).

Ooooooo and like we didn't know this from simple basic PHYSICS and not some BBBMAX super powers.