See
45-70 Scope in the FAQs, there's also a couple recent threads on it and the 500S&W, the BC is a Handi with a 32" barrel basically, what works on one will work on the other, hammer clearance is the key issue as it is with any H&R, there are scopes better suited due to their design, fixed power scope or scopes with small power rings/ocular bells can be mounted in lower rings, plan on high or extra high if you go with a scope with a large power ring/ocular bell unless you modify the hammer which is well covered in the FAQs also.
Tim