Hornady used to have a poster that listed the velocity windows of each XTP bullet they offered. I cannot find mine for the life of me..
IIRC, the .429 240gr XTP had a velocity threshold of around 900fps for expansion to occur. At lower velocities the XTP will expand to at least the length/depth of the hollowpoint skives, or notches.
Its 900 fps. i have the old Hornady chart on my computer.
Its a good game bullet. Since 1999 i've used that bullet in my .50 caliber muzzleloaders. Its killed about 25 deer and a couple hundred wild hogs for me.