Systems and methods applying density altitude to ballistic trajectory compensation for small arms are provided. According to some embodiments, systems to solve ranging and ballistic compensation problems in firearms at various range-distances under varying atmospheric conditions are provided. Such systems can comprise a reticle housed within a firearm scope; and ballistic compensation information associated with at least one of the reticle and an aimpoint adjustment feature of the firearm. A user can utilize the ballistic compensation information to compensate for various density altitude levels to adjust the firearm scope. The ballistic compensation information can be indexed by density altitude level to enable adjustment of the firearm scope to adjust for varying atmospheric conditions and range distances. Other aspects, features, and embodiments are also claimed and described.