Just a thought on your problem with powerbelts. I've had similar problems shooting them out of my M/L. If you read the insert that comes with the bullets carefully you'll see that they are actually under-bore sized. Made to fit a CVA rifle which is slightly under bore compared to your TC black diamond. They suggest fireing a squib round in all other guns before shooting the powerbelts. I'm not a big fan of this idea, to me the only consistent barrel is a clean barrel. Plus if you're getting 1-1.5 inch groups at 100yds with your old bullets why change. Heck they're probably cheaper, and those are pretty good results.