50 and 20MM Projectiles in an Air to Ground and Ground to Ground Role Sources Comparative evaluation of the 20mm developmental ammunition - exterior ballistics Graphical Trajectory Compendium of Caliber. Exterior ballistics of the 20mm rounds of the М61, М197, and М39А1 cannons have been adjusted.Exterior ballistics of the 20mm rounds of the Oerlikon cannon have been adjusted.