First off, Welcome to the forums!

Sounds like you got a nice combination gun there.
Can you get some photos up so we can see it?

Images of the proof marks on the underside of the chamber end of the barrels and of the marks on the action flats will be especially helpful.

What is the action type?
Toplever, underlever, hammers or hammerless?

Show us the sights too.

Sounds like loading for the rifle should be pretty simple, Woodleigh makes a very nice 198gr jacketed soft point bullet for that bore size.

Do you know the chamber length of the shotgun tube?
The proof marks should answer that issue.
There is likely factory ammunition available for the 16ga end of feeding this gun.

Photos!

--Tinker