There have been a growing number of shooters on the http://www.huntingnet.com/forum/
black powder forum that have shot the new Hornady FPB's and virtually everyone has reported excellent accuracy results out to 100 yards.
Maybe only one person mentioned that they loaded tighter than plastic sabots do in his tighter TC bore.
In a medium twist traditional rifle the accuracy group may begin to loosen up somewhere between 50 and 100 yards, but the reports are that they really do shoot great in most all of the modern inlines.
The only real complaint that I've heard is how much they cost at Walmart, about $1 each.