I've used a stainless T/C Thunderhawk 50cal since they came out many years ago. It worked well and gave fine groups with pyrodex and caps. I converted to 209s with a factory kit as described by snicker. I now use 90gr of 777 and either a 45 or 40 cal sabot. These are very accurate and dependable. My Thunderhawk is scoped with an old 3x and crushes any deer within 100yds.
As snicker states, the conversion requires a new bolt as well as breech conversion. I like the 209 conversion, but in retrospect, I think the old caps and 777 work nearly as well.
My Thunderhawk is a very accurate and dependable rifle, and I never heard of any "poor performers"
best, nrb