Go for the shoulder, straight up the leg, preferably high shoulder on axis. Only do behind the shoulder if they're quartering away. Their vitals are pushed up front and high and typical behind the shoulder shots like on whitetails result in wounded animals. I've guided people on axis hunts and taken several myself. Seen them taken successfully with shoulder shots from rifles in 22-250, 243, 6.5cm's, using cheap ballistic tip ammo. I've also seen them liver shot with behind the shoulder shots that would have been perfect on whitetails. Keep it simple, no trick shots, dead center on the shoulder, don't worry about meat loss on a nice mature axis buck. This is what I tell my clients. Good luck!