If target shooting is the purpose for a gas gun, the 6mm or .22 cal is a super way to go. Although the 7mm offers the best bc bullets and would be a great choice in the .284 Win and larger, the smaller calibers really do offer significant advantage in the way of inherent accuracy and little recoil.
I shoot across the course and some F class, but prefer to use a gas gun almost exclusively. I have used the 6mm Hagar and my own .240 Tomahawk with excellent results, but I have permanently moved to the .220 Thunderbolt, which is the .22 cal version of the 6mm Hagar. I allows you to shoot the .22 cal heavies (77, 80, 85, 90) at 3000 fps+, with mid blowing accuracy out to 1000 yds. I have also built several .240 Tomahawks ffor guys who prefer to shoot the tactical style matches engaging targets at 1200 yds. The big advantage of these cartridges is the abillity to load heavy bullets in a mag.
All can be used from a compact and reliable platform that the AR15 is so well known for. Just some food for thought. Not saying the AR10 is not a good choice, but boy, the 15 has coe a looong way baby.
JS