Overwriting a materials file with a datapack has no impact on materials for bullets/parts crafting.

Removed items/tags are still allowed and added ones cannot be used.

Hey, could you maybe send me a test data pack that you were trying?
I have some ideas as to why this might not get applied, but need a test pack

Here's a simple datapack that adds thermal's gunpowder block as a valid gunpowder crafting material: overwrite.zip