Many of the .310 Cadet rifles have now ended up back here in Britain, as a (poor) substitute for the pistols 'bought' from us 20 years ago. Almost all reloaders have found that the use of a softish lead heel base bullet is essential for any accuracy.

I expect a .357 Magnum case could well work, and would hold more powder, but the higher velocity might cause the bullet to strip its rifling. Let us know how it works!