Getting to 600 Yards with a "Detachable Handle"
The single most expensive part on a "Space Gun" used in NRA HiPower matches can easily be the sights. The barrel can be had for $250-400. A good rear match sight can cost $350 with an additional $50 for the aperture, while a front sight can run $100 and $100 more for the mount.
In an effort to keep costs down a, "Detachable Carry Handle", as used on AR-15/M-16 flat-tops, can be used as a rear sight. These cost about $110 with a "match" configuration sight. The front can be a Mid-Tompkins at $40, with a $12 base and a homemade barrel band. While this works well at 100 yards, the elevation screw lacks enough vertical travel to zero at 100 yards AND at 600 yards.
The solution a fairly simple. The sight is dismantled, the base is mounted on the upper receiver, and, using a 1/2-inch transfer punch, the center of the elevation screw is marked on the surface of the upper.
The upper is mounted in a mill and the quill is centered over the center punch mark.
Then, using a 9/16-inch end-mill, a recess is cut about 0.100 deep.
Then the whole thing is reassembled, the rear sight is lower to the bottom, raised a couple of clicks, and the FRONT sight is lowered until a 100-yard zero is obtained. The rear sight now has enough vertical travel so as to allow zeroing at any distance between 100 and 600 yards.