I like the 6.5x284 for Deer. I've shot them out to almost 300 yards and have had them drop in their tracks more often than not. I use my hand loads of 140 gr. Hornady SST Bullets at around 2800 fps.
Last year i shot a large doe through both lungs at 265 yards and it dropped so quickly i thought i missed it. The 4 other deer standing next to it never moved and i had to count them to see if there were 1 less standing.
The Hornady SST bullets are very hard hitting hunting bullets. At the velocities i load them to, they seem to stay in tact for the most part and make a massive wound channel. I also use them in my 7mm-08 Long Range T/C Encore Pistol with the exact same results out to 300yds .....So far.
I think you cant go wrong with the 6.5x284 for Varmints to Most any North American Big Game.