iStrelok is a ballistic trajectory calculator. - 984 reticles supported! Full list see here: http://www.strelokpro.online/istrelok/reticles.html - You can see holdovers on your reticle and scale holdovers to any magnification (if reticle is located in second focal plane). - Trajectory calibration ("Truing") feature. - abitity to change target type and select from the long list - ability to input current altitude from internal GPS sensor - iPhone internal barometer support Small, handy and accurate. Field-proven accuracy since 2001. Supports imperial units (yards, feet, inches, etc).