John, I´ve rechecked the velocity of my 28 and it show an average of 516 fps with Gamo Match pellets 14 gr.
Yes, I had already chronied several different pellets in my Diana 28, but I don´t remember exactly their velocities. Of course, the heavier pellets showed slower velocities. But if you wish I might chronie the pellet that you want: JSB exact, JSB Straton, Gamo Magnum, Gamo Master Point, Gamo Hunter, RWS Superpoint, RWS Hobby, RWS Superdome, RWS Meisterkugeln, Crosman wadcutter and Crossman Copperhead pointed.