RE: bullet Question
I don't want to give a bad review of the Powerbelts because conceptually, I think they're a novel idea. Also, I know plenty of people who have had luck with them.
However, having just gotten into MLing this summer, I spent quite a bit of time at the range with several load combinations. The first of which included Powerbelts. I'm shooting a T/C Black Diamond XR, which is pretty reliable with a wide range of loads. However, I think I could really tie my initial consistency problems to the Powerbelts, especially at 100 yds.
Again, it's not a bad bullet, and with time, I could probably find the right power combination to keep it consistent, but it was getting expensive. Your question was whether they were basically interchangeable with T/C Mags. My answer is a stout 'no, absolutely not'.
With MLs, I wouldn't expect any two loads to act the same until I proved it.