Well, you can shoot all 3 heads of each delt fairly easily. The anterior deltoid has the most nerves in it and a major artery (assumption. if anyone knows what that vein is lemme know). The lateral delt is the most common area for injects, you want to aim in the middle and inject perpendicular to the muscle. The posterior delt you sorta have to reach over with the opposite arm and insert the syringe in a downward motion. I find the rear delt to be a very ideal yet seldomly used location. 25g 1" is a decent size needle to use.