Don't give up the ship on the Hornady XTP's just yet.
My Omega shoots the 300GR XTP's, SST's really well and at several different loads.
Ensure that your barrel sparkling clean and dry to start.
If this gun is new, you need to clean all of the packing lube from the barrel. Use a bore solvent followed with alcohol and then dry.
You may need to spit patch/dry patch between shots. I also follow up with an alcohol then dry patch between shots.

Select a different powder (Black Mag'3, 777, or Pioneer).
If loose powder start at 80grains and work your way up and settle with what shoots the best. (Loose powder BTW, will live/last longer than the pellets will).

If pellets, start with two 50 Grain.

Let us know how you end up.