i have used powerbelts.. they work well in that rifle.. but the best ones i have ever used as far as acuracy have been the CVA deerslayers. 300 grain.. i am not sure if they make them anymore though.. i will need to do some recerch. its been a few years since i have used the 50 for hunting. and have only used it for target shootingand competitionshooting and most of that is done with round ball.
looks like some places still sell them. by far the best acuracy for a conocal from my cabelas hawken
http://www.impactguns.com/store/043125115914.html