Some folks don't like to use the MatchKings for hunting, especially if you hunting deer. One of the advantages of the 6x45 is that there are heavier, more traditional hunting bullets than in the .223 Rem.
John
Also there is a great article floating around from Steve Timms on the 6x45mm. It was in Handloader years ago and it is reprinted in "Wildcat Cartridges I&II" by Wolfe Publishing.
GregW: Regarding RL-15, check out Steves Pages at http://stevespages.com/243_9_80.html for the 6mm-223Remington with most bullet weights. Steve says that the powder he has hi-lighted for each bullet weight is the one that he recommends for best performance. Of course this is his opinion, but I just bought 2# of RL-15 for my AR15 with 22" SS Lilja barrel with 1/9 twist. I want to push 80 gr Remingtons and 85 gr Nosler Partitions.