I have a .357 Mark I. Friend has a VII .50 with a .44 Mag bbl. They're fun guns. We've run them at Pin Matches.
Reloading for the Desert Eagles: Reduced Load is not a goal. They're Gas Operated, and they have twin double-coil recoil springs. You need a hot load that generates a lot of gas to cycle them reliably. Also, don't cheap out and get soft plated bullets. They'll lead up the barrel and/or clog your gas port, and bang: you just made a single shot. (I nearly ruined my Mark I doing just that.) Good thick jacket is a must. You won't be putting thousands of rounds with it so splurge a bit.
Now...all that said, there could be a powder out there now that generates the amount of gas needed without the associated high pressure and felt recoil. But, I run H110 in my Magnums (including revolvers) and part of the fun of the Eagle is seeing the HUGE fireball out the end (or in the case of the .50 and .44, the big RINGS OF FIRE that go down range) and the concussion of the blast.
This is a gun that AH-nold uses to shoot airplanes down airplanes flown by robots, right? Reduced load, indeed.