Sorry no body answered you, I will try.
1. a bull barrel will be about .810 and a regular barrel will be about .650
2. All Handi rifle scope bases are the same, they all will fit.
3. About a half inch, depends on how far down the barrel you are talking about. Half inch at the breach.
4. They do not have extractors, they have ejectors. I have never heard of a stuck case in a Handi RF.
Larry