Gents:
A good mate, AkMike, alerted me to this thread a few minutes ago.
I picked up this 12 bore Whistler a few weeks ago and am waiting on a ball mould and sizer. A barrel caste shows the diameter to be about .748" Chambers are 2 1/2", barrels 26 inches, and the weight is 11 1/4 pound. Rifling twist is 1:100 for a round ball. A gent on Accurate reloading send me 100 plastic hulls and I have two boxes of Mag Tech 12-bore brass cases and an RCBS reloading die set. I do have a Lee Precision mould but it is undersize but will fit in a shot cup of a wad and slip into the cases. The difference in wall thickness between the plastic brass shells is about .005-.007". It appears this rifle was made for brass cases as the shoulder at the end of the chamber is very slight. Should be fun!
Cheers,
Cal