It is not the rifle. I have the same rifle. I am going to make a guess and suggest you were pushing the PowerBelts too hard. Most suggest a 80 grain max load. Also ditch the pellets. It is much more cost effect to shoot loose powder. Now that Hornady XTP is a good bullet. From all I read it is a real deer taker. With my Optima I shoot a 260 grain Scorpion Funnel nose made by Hornady. But the funny thing is, I have never taken a deer with them. Not from lack of trying mind you. It seems when I hunt with an inline I never see anything. I take a traditional round ball rifle out and always see deer.
Here I my suggestions. Get loose powder. My Optima likes 90 grains of powder with an XTP. Shoot that at a deer and then let us know what happens. Now I have shot a lot of powerbelts. And I have seen a lot of deer taken with them. I find them very accurate. But I do think there are better bullets. One of my favorite bullets (but they are expensive) is the Barnes MZ 250 grain.