that sounds like the accutrak model which has a bullet drop compensator built in. You have to take the screw out of the top of the knob, I think it is allen head but could be mistaken. You then adjust the dial for elevation with cap removed and sight in to be dead on at 200 yards. replace the cap with the 2 being 200 yards and against the stop so it will not turn down any futher. You can then dial up to 600 using the numbers on the cap. The trajectories for different rifles were compensated for by using a few different plastic rings that fit under the cap too. You had to pick the one that matched the caliber you were shooting and install it. I had that scope and if I find the owners manual I could probably tell you what calibers for which ring, but if you bought it from someone with similar caliber you should be alright and if you don't have the other rings you won't have much choice anyway. Surprisingly it worked well on my rifle and I liked the stops so I could adjust it in a dark blind and return back to my zero, the glass just wan't the quality of some of my really recent scopes. I wish more manufacturers would make a simple bullet drop compensator with zero stops that would work out to about 800 yards with one turn on dial. So far I have only found nightforce which is expensive and heavy.
