Landon,
There is alot more to shooting a 6 in group at 1,000 yrds. than haveing a gun that will do it. With a 180 gn. Sierra SBT bullet at 2,600 fps, which is abt the max. velosity you will be able to get from a .308, the bullet would drift 98 1/2 in at 1,000 yrds. with a 10 mph cross wind. If the wind dropped to 9 mph it would drift 9.85 inches less and an 11 mph wind would cause it to drift an extra 9.85 inches. With any variation in wind speed it gets real tough to shoot small groups at 1,000 yrds. I would recomand you forget 1,000 yrds. for now and learn about 500 yrds. first where a 10 mph cross wind will only move your bullet 20.42 inches.
I have a gun that should come close on a good day. Is a .22-6mm. I`m pushing an 80 gn. .22 cal bullet at 3,475 fps. With it drift should be around 87 in. at 1,000 yrds. I`ve got alittle over $ 1,000 in the gun and another $ 1,050 in the scope. This was built as a coyote rifle for use from 250 yrds. to 800 yrds or so. If I were going to build a rifle for serious 1,000 yrd. shooting it would be in 6.5 cal. Minimum a .284 Win. case.