H4895 will be the powder of choice for cartridges with a bore ratio similar to 7.62 NATO and 30-06. It has been tested by the best. There is a hint in there.
W748, W760 and other ball powders with similar burn rates.
Another hint. Gas ports are about mid barrel length in military arms.
Mid to heavy bullets within the particular cartridge would help also.
Bore ratio of the .243 Winchester ? You might have to live with what you got.
