RE: Factory loads for 260
The 120 grain bullets in 6.5mm (.264 caliber) have a sectional density similar to a 165 grain .30 bullet. So the 120' s are plenty capable of sufficient penetration on deer size game. Which bullet in particular depends on a number of factors. I have had very good success with Remington factory loads using the 120 Nosler Ballistic Tip. It is a very accurate load in my Remington 700. It has worked very well on the Pennsylvania whitetails I have used it on. (Being from Georgia I am going to assume that your whitetails are roughly the same size/weight as Pennsylvania whitetails.) Though this particular bullet is very accurate for me and to this point has provided both adequate penetration and great terminal performance it is a little " soft" . I do NOT think it would shoot through a whitetail lengthwise to reach the vitals. But as long as you pick your shots directly into the chest through the ribcage...it has been devastating. If you think that you might need to take a more " angling shot" I would be inclined to try some " tougher" bullets. Perhaps the 125 Nosler Partition? I have had considerable success with Partitions in all calibers I have tried them in. And they will no doubt penetrate from any angle.
Good luck on finding your load!