I tried customizing the weapons, but whenever I create an instance of data/modid/etc. it fails to load, if i remove data folder, the pack works fine. I can't even customize vanilla weapons.

Here is an example json i tried using:
{
"attributes":
{
"two_handed":
{
"armor_negation":5,
"impact":1.5,
"max_strikes":4
}
},
"type":"spear"
}

Tweak two_handed to two_hand