the 243 is a very nice gun to shoot. u can reload and make it a real barn burner. lapua brass is a dream to work with, or military brass (308) can be necked down for almost nothing. but watch ur pressures.... mil brass has less capacity.
wanna go a little faster, go with the rem 6mm..... but no luck on good brass.
the 243AI isnt exactly an off the shelf chambering, but is pretty impressive if u know how to feed it right, and uses 243win brass, with better brass life and much better ballistics. also it will still shoot off the shelf 243 ammo if u get in a pinch.(my next gun will be 243AI)
even further into the big-6's is the 6-284, but its quite a bit ahead of the average reloader.
all the 6's are explained on http://www.6mmbr.com/index.html
anything between the 243 and 7mm ultra mag is what u need.... depending on how much u wanna get kicked.... in the shoulder and wallet
id look into the savage model 12's or 10's