What caliber and what twist are we talking about?
Mini, maxi, and REAL bullet moulds are available in many calibers and bullet weights.
Check with Lee Precision for their line of ML bullet moulds.
If your rifle has a slow twist of 1 turn in 66 inches, a short mini or REAL bullet might work. If the rifle has a faster twist of 1 turn in 48 inches, you should have a wider range of bullet choice.
I don't know what import laws you have to deal with regarding ML projectiles, but you can order several different bullets styles and weights, to test for power and accuracy, from companies in the U.S.
Some sellers on e-bay offer quite a few different bullet styles too.
IS simply buying a second barrel for your rifle an option?
A .530 Round ball, for use in a .54 cal rifle weighs right at 230 gr.
J.D.