Hmmm. Been looking at that.
The thinking continues. It's all about the bullet and the speed you want to deliver it at. I have established that the GSC 75HV is not available, only the 62HV in .243. That's straight from Teimens in Holland who's also questioning reliability of supply generally due to the recent doubling of copper prices. That's a whole different issue.
If I want to make this future proof the limiting factor is going to be the choice of monolithic bullet. Barnes TTSX only go down to 80gr as does the Hornady GMX. Nosler is 90gr. So I'm going to have to build around an 80gr monolithic.
I know they work down to around 2400/2500 fps so if I want to be able to reach out to 300m I need to be launching at more than 3250 fps. The 6BR is not going to achieve that. More like 3050 fps which I don't have direct experience of so am not inclined to trust.
The Dasher should boost that to around 3150 fps, so it's close.
@srvet has suggested the 6XC which is interesting, with Brass available from Peterson and Norma. Dies seem easy to get from Spud and it hits the 3300 fps requirement. With all the records it's chalked up and the championships it's won it ticks the accuracy box as well. No hint of any feed issues either.
6mm-6.5x47 is another option. I can't see any point in going 6mm Creedmoor or 243win as they just consume more powder and make more fuss whilst achieving no benefit.
The lead bullet will be the 103 ELDX and that should fly at around 2940fps from a 22" barrel on a 6XC which is fine. Another great benefit is that both bullets will work well with RL16, so no temperature sensitivity from the loads. I can't use RL16 in both 6mm-6.5x47 loads.
So everything is pointing at the 6XC. Be interested to hear any experience of that as a chambering if anyone has some, especially if it's in a shortish barrel (compared with the 28/30" ones the competition guys are using).