I have a T/C Black Diamond XR that has been shooting 2" -3" patterns with T/C mag sabots at 100yds for the past two seasons. This year I couldn' t hit a 2' x3' piece of cardboard. I moved up to 50 then 25 yds. finally hitting the target, but most of the time the bullet hole was oblong not round, suggesting the bullet is tumbling. I am using the same bullet and same charge(100 grains pyrodex pellets) as previous years. The barrel was thoroughly cleaned before putting away last winter,and I looked at it before shooting a few days ago and it looked perfect. Since I was having no success, I tried some 295gr. powerbelt, and was able to get a 6" pattern at 100yd. Any thoughts?