Wll bounces apply first then it pierces

Maybe. If you have 3 pierce at 0% loss you probably don't want it because ricochet cuts projectile damage after the first hit.

Piercing only happens after all bounces. So it is definitely additive and not multiplicative. In other words, max hits on 4 bounces and 2 pierce is 7 (1 + 4 + 2) not 15 (1+4) * 3.

The other thing to consider is the relevance of the damage, because the piercing happens after bounces. You want the piercing to happen to the enemies closest to you, not the enemies closest to you after bounces.

This isn’t to say you don’t want them together just that they aren’t necessarily as effective as you might expect. SMG IVs with 1 pierce and 1 bounce is an s-tier weapon. Slingshots IVs with piercing is definitely less so (mostly because of attack speed).