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
I use Reloder 22 for the 100 & 105 grain bullets in 243.
Nice case fill, good accuracy.
Trouble is its not that easy to find now, so as with all my other loadings I'm moving over to Viht (good availability) & Reload Swiss (good availability, good full product range, & at the moment good price - I suspect that the Swiss will soon go up to Viht prices now they have started to get accepted in the UK & are starting to supply it in fancy bottles. I got this from Nigel the importer a few months ago - wondering how things are panning out).
im using vit 165 for 105 grain bullets I will get 1/2 groups but that's in a 1-8 twist barrel. im not sure how you will get on with a 1-10 twist barrel for 100 or 105 grain bullets but I may be wrong give them a go and let us know .