Powders in the burn rate of Rl-19 and 4350 work well with the 120 gr pills in the 260 out of handguns.
R-22 and H-4831 are also great powders but muzzle blast will be a little more with these powders because of the slower burn rate.
Just use data for the rifle loads but I always lean toward the medium burn rate powders for the round instead of the slower powders recommended for a certain round.
Generally velocity is basically the same and muzzle blast and flash can be reduced a little.
Do not make the mistake of thinking that a very fast powder will produce better velocity in the shorter barrels, use the proper powders recommended for the rifle load and these will still be the top performers in the handgun.
Velocity wise, you will get into the 2600-2650 fps level with this round and the 120 gr Ballistic Tip on average. May be slightly less or more depending on specific barrels.
If your looking to build an XP in this round, you may want to consider the 6.5mm WSM instead. This round will drive the 120 gr Ballisic Tip out at around 3050 fps out of the same length barrel.
The XP I built myself fitted with a Holland QD brake recoils dramatically less then a 7mm BR and the muzzle does not lift at all.
Just an idea, if your buying a factory XP then the 260 is a great round.
Good Shooting!!
Kirby Allen(50)