Having thought about it for a bit, i’m thinking the solution is twofold: 1, give HP ammo a healing reduction/negation effect, this would make it better for PVP and give it an edge against irradiated zombies. 2, make more high-tier/boss-tier zombies durability come from raw health numbers as opposed to high armor rating. This makes the choice between the alternative ammo types more nuanced: both have general horde applications (AP with over-penetration, HP with a damage kick), both have applications against high-priority targets (AP hammers armored zeds, HP would make killing irradiated zeds much easier).
This seems like the most straightforward change to HP ammo to make it competitive with AP.
This seems like the most straightforward change to HP ammo to make it competitive with AP.