If I had your rifle... and I don't. But I would like to have it... If I had your rifle, I would get me some 200 & 250 grain T/C Shockwaves. I would start my first loads with 100 grains of powder 2f grade. Pellets or loose. I would swab the barrel between every shots. One wet patch and followed by two dry patches. If I got acceptable groups then (which you should) I would push the load to 120 grains of powder and do the same thing all over again. There is no reason why that rifle should not handle 150 grains and not give excellent accuracy with pellets. With loose powder I would stay around the 120 grain area. I base these guesstimations on different loads I have ran through my T/C Black Diamond XR. Although we have different actions, we have similar steel in the barrels and the same twists.

As for the strange groups you're talking about with different projectiles, that does not surprise me at all. I have seen similar weights of projectiles shoot as much as 6" different in groups. Decide what you want to hunt and what projectile you want to use and then sight the rifle in for that. That rifle is a 200 yard rifle WITH PRACTICE and I am sure you are going to practice before attempting such a thing. The Shockwaves are a good projectile. The one thing I have read in different posts on the Encore that surprise me is some claim the Shockwaves will not load and then they shoot Powerbelts. I would think with what you're paying for that rifle, it should shoot anything out there. If the Triple Se7en powder does not work for you, try some Pyrodex pellets.