Impossible to answer without knowing the model of the rifle.
No published sn data.
10-100 usd
@1979-1988
200
No published data I know of.
Rossi Customer Service in Florida- Phone: (305) 474-0401
100-300 USD
Follow instructions in owner's manual
H&R and I believe Rossi have single shot rifles with interchangible barrels.
50-125 USD
The tube that slide in to keep the bullets in