i am shooting 777 in my .54 renegade strictly with roundballs and #11 caps. no misfires or hangfires yet, and great accuracy with lighter charges. i tried it with a musket nipple, but the hammer was blowing back to halfcock with each shot, even with loads as light as 80 g. FF 777. so far i have had no blowbacks with the #11 caps, but i dont go over 90 grains of powder. with the steel buttplate, recoil becomes pretty uncomfortable with 777.
keep this in mind, though, you do have to clean up the 777 fouling pretty much right away. i made the mistake of letting mine sit for a few days before i cleaned it, and the fouling turned into concrete. i honestly thought that i had lost the barrel, because the fouling felt like corrosion in the barrel, and i couldnt get it out. then i got a 20 gauge bronze bore brush and some hoppe's #9 benchrest cleaner and scubbed the heck out of it. that bore brush was a lifesaver, and i have a slick bore again. but it scared the crap out of me, i thought i'd lost the old girl!