If you are buying factory, then 6mm remington. If building, then a hot 6mm or .25 fit the bill. For 6mm's think 6mmAI, .240 wby, 6x284. All easily done by any reloader. For hot .25's think .25-06AI, .257 wby, .25stw. The last is a wildcat, but will throw a 100 gr bullet 4000 fps. Had to mention that. Now for the real question, what are you going to build or buy? Look at richardscustomrifles for some ideas.