RL22 or N160 (very similar burn rates)
N160 is cheaper, easier to get and readily available in Hannams warehouse the last time I spoke to them
H4831 would match RL22 and N160 for burn rate but is hard to get also
N550 is double base and not really best suited to overbore cases (which ,243 is marginally described as, large volume to bore ratio) could
Varget is a joke, you may as well give up shooting!
N140 is possible but will leave you with less than ideal case fill ratios whilst working out slightly cheaper on charge levels
By the way a 600cm barrel is exceptional....mm I hope