Can anyone guess the velocity of a 21.5 grain pellet in a Diana 34?
You can shoot them, but they won't give you the best overall performance (Highest FPS & FPE) out of the Magnum Diana Springers. They are gonna be harder on your gun too. They are a good choice for more powerful (ie: PCPs) air rifles.
I have 2 Ruger Airhawks and they both shoot well using CP Ultra-mags 10.5gr. They also shoot very well using JSB Eact Monster 13.43gr. They just seem to shoot heavyer pellets better for some reason.