hey Robby, on a magnum gun like that you would be robbing yourself by getting a .177, thinking that you'll have a flatter trajectory. I imagine that long distance is what you want to shoot and i can assure you that long shots in .22 will be probably flatter than the .177, as ballistically the .22 ammo will retain not just more energy but at the end will shed velocity at a slower pace. Not to mantion that unless you are shooting the 15 gr .177 you may go supersonic with every shot (rimfire loud) and that while the pellet will leave at supersonic speeds, when it gets pass 50 yards it will be again subsonic, meaning that it will have to pass through that Transonic stage when pellets become less stable in flight, (they will distabilize as they pass through it).
All that "transonic" talk is completely knew to me. its that stage where your pellet goes from flying at supersonic speeds to then going sub sonic. The actual "change" speed. When a diabolo pellet goes through it it distabilizes its flight.