I saw a post on 6mmbr.com about a new 6.5x47mm Lapua cartridge they've apparently developed for 300m competition. The suggestion was made that necked down to 6mm it would perform very well against the 6XC (I think that's the Tubb cartridge) with the 115gr DTAC. It would probably do very well with the 107gr MK too. Apparently it might take a large rifle primer but have a small (60 thou) flash hole. I hope there's some truth to this.
Ben