Does similar equipment stack in Resident Evil Resistance?
Yes, equipment do stack.
I confirmed using FMJ Rounds on Becca.
Here my results after several tests on practice mode. Damage on zombies.
FMJ Rounds II : Highest weapon damage was 446 FMJ Rounds III : Highest weapon damage was 462 FMJ Rounds II & III : Highest weapon damage was 503
As you can see, same equipment stack with each other. Iron Nails should be the same but on melee damage.
I hope this helps.