First let me qualify my response by saying I build rifles and have built well over two dozen 6.5x55 guns and own a dozen or so at this time. I also own five 260 guns at this time, that I have built. I shot silhouette for 25 years and shot on the 101st Abn rifle team.
There is one very real advantage to the 260 over the 6.5x55 and the 7x08 (I also have owned a few 7x08s) The 260 is more accurate than the 6.5x55 and the 7x08 in the finite accuracy stage. It is capable of .10 to .14 inch groups which the 6.5x55 and the 7x08 will not duplicate. There is no reason that I can determine for the 260's accuracy, as compared to the others, but it is what it is.
If you are working with the 260, try H-414. H-414 produces the best accuracy for the highest velocity in the 260. Bullets; there is not much reason to use anything but the 120 Sierra HPBT MK. It kills and punches paper with the best. At 3200, the 120 is supoersonic past 1000 meters. H-414 will deliver 3200 fps in the 260 from most guns if you work at it.
In the AR-10T the 260 with the 120 MK at 3200 fps is a real terror in a military rifle match. I also owned a 10T in 260.
Go study the 260 and the 6.5x06 and the 6.5x284 carefully. You will discover some efficency magic. The 260 will in many instances out perform the two bigger capacity cartridges in velocity, especially with lighter bullets and it will come to within 100 fps of the bigger boys with the 140s. I also own two 6.5x06 guns.
So if you you do not understand, join the crowd. I was going to build a 6.5x284 back years ago, before they were popular, to go with the 6.5x06 guns, but got to looking at the data and decided to build the 260 instead. It was like eating potato chips. My last venture was the 260 XP-100 which is real thumper. It is a crowd pleaser at the range, they applaud when I stop shooting it.
I am a 6.5 kinda guy.
Best,
Ed