sakmyk:
If you don't have easy access to cerrosafe, you can chamber-cast fairly accurately (within a couple of thou) with sulphur. Our local pharmacy sells it in a 100g container labelled 'Flowers of Sulphur', which is simply sublimed sulphur B.P.C., used as a mild laxative (!). Melt it in a tin-can and pour it into the blocked chamber as per cerrosafe. It tends to shrink as it cools, so you need to keep 'topping-up' the chamber till it fully solidifies, but if you do that, the resulting cast is fairly accurate. I've never used cerrosafe myself, hard to get in Australia too, always used sulphur.

The Greener 'Empire' has a very long action bar for a 12-bore shotgun. I have three of them, two are shotguns (an E-10 and an E-17 grade), and one is a converted .577 x 2 3/4 NE. They are all 2 1/2 inches or 62.5 mm from the standing breech to the centre of the hinge-pin.

Pity you can't get that rifle shooting, load development would be a heap of fun!