If you change the item.useStyle to 5 that usually does it since thats what boomerangs often use (the useStyle is actually called 'Shoot'), and change the item's item.shoot to the projectile you want to shoot (ProjectileID.ITEMNAME or the ID number).
I havent fully coded a projectile yet but there are videos on youtube for certain that help.
dont forget to also change item.shootSpeed for the distance it goes