I do not think it is my chrony because my pellet would have to be traveling at 1200 fps to get the trajectory that I am getting according to chairgun. My chrony is only 4 months old and I have never shot it (this one anyway). I am shooting 18 grain JSB pellets and they are not breaking the sound barrier. My Marauder is stock except for a Marmot Militia hammer forged barrel. I doubt I could get an 18 grain pellet that fast. I have a UTG 6-24X56 10X half mil dot scope and I use the Hawke 10X half mil reticle for both programs. It is very clear that I am on 20x magnification with my scope. To get my scope height, I measured the distance between the top of my shroud and the bottom of my objective on the scope. I then measured the diameter of my objective, divided it by 2, measured the diameter of my shroud, divided it by two, and added those two numbers to the distance between the shroud and objective. As far as BC, I'm using what the programs say it is for the 18 grain JSB's. Strelok asks for twist rate of the barrel, so I contacted Marmot Militia for that number. The new barrel is the same length as the factory barrel, so that shouldn't be my issue. I guess I'll try to figure out my own BC. Might have to take a math class for it though.