Some more info on the Savage 212. I looked at one,
got more measurements. The port is 3.812" long.
Can be made longer. Bolt travel is 4.625" and can be .50"
longer. And with full diameter bolt can have extra
lugs in back. The Mag box has room for extra length in
front of it and behind it. It could work for 12ga FH
and 700HE long case.

A fellow on Gunsmithing forum found a way to make
jacketed slugs using copper tubing caps, the solder ones,
by putting them in a die and using a shaped punch to form
a round nose copper jacket, that can be filled with lead.
He made .700 cal ones, different dies/punchs could
be made .729, 12ga size. I got a few here and going to see
what is possible. I got bigger caps also to see about 4ga
and 8ga ones. Here is picture of the CBC break action with
the 20ga heavy barrel mono-blocked in, with action open.Ed