do i need to register my assault banjo?
Depends. If it's a little short tenor 4-string, it may be concealable, and it may not need to be registered, but you may have to be licensed.
If, on the other hand, it's a 5-string such as those Bluegrass and country-western gang thugs favor, there may be no law requiring registration, and not even a law requiring a license to purchase. But remember it's not really a sporting instrument, and please don't abuse it.