Re: Which chamber, 6mm or 243 ??
Check out the 6X47 Lapua. It is a necked down 6.5X47 Lapua. The cartridge offers some real advantages. Namely, superb, high pressure brass, ppc type accuracy, and little recoil in a very efficient case design. Reported velocities are 3200 with the 105 a max. The cartridge is dominating F class.
There is a great article on the cartridge on 6mmbr.com. From the home page click on the 6mm topics, you will see it.