I tried a few in my 21" GM 1:28 barrel today using 80 gr ffg T-7 and they looked promising. I'm switching to fffg Goex next trip out, though. Just a little hesitation on ignition with the T-7 and I was using musket caps. Back to real BP & #11s for me I think.
I usually start off around 70 - 80 gr with a .50 cal.
So far everything I've tried the 250 gr REAL in has been acceptable on accuracy. The 1:48 T/C NewEnglander really likes them, the 1:28 GM barrel seems to like them, and the 1:28 inlines weren't great on accuracy, but it would be plenty good enough for a quick load for a follow-up shot if needed. Even the Lyman Plains Pistol seem to shoot them well.