For bfheroes the projectiles are usually defined in their respective host vehiles tweak file. I did modd the tank buster to shoot soccer balls so I should know a bit about this.
For example the projectile for the Sherman tank should be found in the tweak file for that tank. (its called "BRI_Tank_Sherman_Barrel_Projectile")
The projectile template name is "GenericProjectile" so just use that when searching with Notepad to find the projectile template for each respective vehicle.
If you get the projectile name wrong when changing the projectile the vehicle/handweapon firearm will fire then you'll get the issue of it not firing anything. :P