The shotgun seems to be doing far too little damage to Big Biters. Standard shells are doing about 2 dmg at point blank range (about 1/20th what the statistics indicate).
Piercing shotgun shells are doing even less; 1pt of damage at point blank range (sometimes absolutely nothing). The behavior seems about the same between the shotgun and combat shotgun.
Damage/armor calculation problem with the shotgun shells? It almost seems like the shotgun blasts are healing them? The shotguns seemed to behave fine in 0.7.1.
I've attached a savegame with a car, both shotguns and shell types, and some big biter test subjects wrangled up to kite and shoot at if it's helpful.

Shooting speed is upgraded to level 3 and shotgun dmg to level 1 iirc.