I have two questions one is obvious what is the barrel twist of the Remington XP-100R in 260rem and my second one is what is a good starting load(having never shot a rifle caliber handgun).
Not sure what the twist is I have a xp barrel that I just took off and installed a new barrel for a customer the factory barrel is a 260 rem I will check the twist for you in the morning.
As for loading start just like a rifle with rifle loads and work up to the best accuracy load just like a rifle.