Re: .260 in an AR Platform
Good thing about the AR platform, you can have a single lower, and you can change caliber and barrel length just by switching out uppers! If you are sitting on the fence, you can always get a "308 Parent Lower", and whatever .308 Upper configuration that fits your needs, and shoot the hell out of it. Then if you think you need the extra performance level of the .260, you can always just pickup another Upper and you are ready to go. While I was running up to 5 different rifle configurations at one point (16"-24" barrels, .308 & .260 calibers), I was only running two Lowers (Carbine & Precision Long Range setups), and really could have only run one.
IMHO, a .260 does its best when it is being launched above 2,750 fps where it can take full advantage of the 6.5s great BCs. When you start to get into autoloaders, it can be more of a challenge to hit the higher velocities. If the Upper is built properly, proper barrel length, and the ability to regulate the gas, then getting above 2,750 fps is not really a problem. However if you go with a shorter barrel, and try to run hot/fast loads, then chances are you will "over gas" the system and beat the hell out of everything.
The other issue you have to deal with is the heat in the system under rapid firing. You either have to stay off the trigger and avoid it as much as possible, or learn how to deal with it. If you are going to be running the rounds quickly, then look at slightly dropping the powder charge in the load and making sure you are using the least temp sensitive powders. You also have to be able to recognize when the heating is coming into play and adjust your shooting accordingly like compensating for the rising velocity / higher point of impact.
As many have pointed out, a properly built Upper, with the correct load, and shooting techniques, will shoot very well. However you can't just grab any Upper and Load, and expect to get a "Bolt Rifle Performance Level" out of an Autoloader.
Best of Luck,
M Richardson
