If it's a Hern barrel it is probably 2 1/4". 1f or cannon work well.
Definitely needs pictures. I've taken mine out to the south site. If you go in a ways there are 220 to 240 yard ranges with a nice ridge behind so you have a fair chance of finding your rounds. At four pounds of lead a piece it is worth doing. If you're shooting rounds the weight should be less.
Steve