Get yourself a solid range rod for load development.

Also, ensure that you have removed all of the packing lube from the barrel. I use a quality bore solvent for this task like Butch's Bore Shine. 20-30 strokes with a brush and 20-30 strokes with patches.

Also, spit/damp patch, alcohol patch and dry patch inbetween each shot.
Try to ensure that the barrel is clean and bone dry.

The sabot is likely too large and as soon as you start getting any fouling buildup, then the more difficult loading begins. The xtp should not be that tough to load. The SST or Shockwave albeit a fantastic projectile is tougher to load.

I also like to shoot at least 40 lubed conicals and wads at moderate charge to help break in the new barrel. After shooting 20 conicals, use the bore solvent to remove any leading and continue. This is just something that I've found to help get a barrel broken in versus reaching for the lapping compound first thing.