Try some Federal American Eagle 158gr .357mag. Gets 1500fps with my Ruger Blackhawk 6 1/2" barrel. Fed Fusion same thing with a little hollow point.
That's as hot as I want to go.
I think you need to check the accuracy of your chrony. Factory spacs for the American Eagle 158 gr 357 Magnum is 1240 fps
. Some manufacturers are pretty close to their specs, many are actually lower
, and some of the hot loads from the boutique ammo loaders actually read a bit higher. That's a bit
higher. I doubt that any common brand will produce velocity of 260 fps over their claimed numbers!