Back in 2006 I had a 4" M629 and developed a .44 Magnum load with the Hornady 180gr HP/XTP using Power Pistol. I wasn't trying for a fire-breathing magnum load, but was looking to duplicate an earlier CorBon SD load that used the same Hornady bullet at 1200 fps. It's a nice load with 575 ft-lbs of energy at the muzzle, while still allowing quick follow-up shots.
From the results of my chronographing, and correcting for your 6" barrel, I'd recommend around 11.4gr of Power Pistol at a COL of 1.600". Of course Power Pistol will work just fine with standard large pistol primers, but I'd use a firm roll crimp to ensure complete powder burn. QuickLOAD says this load will fill just more than half the available space in the case, so a double charge of powder should be noticeable if you're paying attention.
Hope this helps.
STANDARD DISCLAIMER: These loads were safe in my gun, but may not be in yours. Never trust any load data from the internet including (especially) mine. Start low and work up slowly, looking for signs of excessive pressure. You are responsible for your own health and safety.