My opinion is that your ML problem is the load or loading procedure. Make sure that you seat your bullet the same way every time. DUMP THE POWERBELTS because I don't think they seat or seal as consistently as a sabot. At least two of my hunting partners, one shooting a Knight and the other a TC Encore, have found that the Powerbelts are VERY inconsistent. When they changed to 250gr Sabots their MLs became much more consistent.
My profile picture is the buck I took on 11/25 this year with my TC Encore .50cal ML and 4-9 Bushnell scope. I'm zeroed at 100 yards. He was about 120 yards out and the bullet hit him exactly where I aimed. I was loaded with 100gr (2 pellets) of American Pioneer powder and a 250gr Hornady Sabot. I used a run-of-the-mill Winchester primer. I should mention that the powder is over 5 years old. He went straight down after he was hit, never to take another step. I love my Encore and except for an occasional outing with a M94 .30-30 is the only long gun I hunt whitetails with.