Yesterday while attempting to shoot 500yd targets, I found that I ran out of scope adjustment. The backstop is a 4'x8' sheet of OSB board and I stapled two targets on top of a couple 1,000yd targets someone didn't pull down after shooting. I stapled them down about 10" from the top of the board. I'm shooting an Ultimate Firearms muzzleloader, not a centerfire. The scope is a Leupold VX6 3-18 with a custom turret matched to my loads. With a 200yd zero setting of the turret, its dead on at both 300yds and 400yds. The turret has a maximum of 450yds marked. Long story shorter, I maxed out the turret adjustment and backed off one click, then sent six rounds. I didn't hear the bullets strike the backstop. Because the range is still wet, you have to walk to the targets, which I did and found that I didn't hit the backstop. After walking back and settling down, I sent six more rounds holding at the top of the backstop. Each round hit the backstop but, very close to the bottom. Ok..... I understand to shoot that range, I'll need a different base. To those with lots of long range experience, will a 10 moa base give me enough elevation or, should I go to a 20 moa base? I don't want to go so far with a base, that it prevents me from a 100yd zero. I'll very much appreciate some long range shooters suggestions.