from all my observations as an RSO and watching the local club ml range shooters, 777 will ignite reasonably reliably with a #11 cap (most seem to use CCI) if the nipple, drum and breech (particularly patent breeches) are kept reasonably clean. the issue of a clean ignition system can also be true of black, but in my experience it's more so true with subs.
<on soap box>
real black powder and subs ... in the past i'd spent more than a bit of time testing and comparing a variety of sub brands (pyrodex, 777, clean shot/app, blackthorn) against real black powder (goex, swiss, kik, elephant, schuetzen, and last year some olde eynesford) in a borrowed and very reliable caplock. bottom line for me - i find that bp is just more ignition reliable than any subs using cap ignitions. however, since i prefer flinters, that eliminates the use of subs altogether because bp ignites in the pan and subs almost never will. flintlocks demand bp, and while caplocks don't have a pan, their reliability (and in my testing and observations, their performance) are better off being stoked with bp than subs. subs really have nothing in common with real black gunpowder. they are fast burning, expanding gas, deflagrating propellant, whereas bp is basically an explosive made of but three ingredients - sulphur, potassium nitrate and charcoal. one also needs to be careful that any particular trad ml will be safe to use subs. i'm not at all being elitist or snobby about bp vs. subs, that's objectively just the results of my testing that essentially views the reliability of ignition, and both fouling and cleaning issues. now i know some folks can't find the holy black locally because their LGS has ordinance issues storing bp, and not with subs, but it can be ordered online from more than a few very reliable sources such as maine powder house and powderinc. yes, there is the hazmat fee and shipping to deal with, but typically both can be avoided by ordering out a 25# lot - split it with yer buds or club. most folks won't go back once they've tried the "holy black". might be you, too - give 'er a try.
<off soap box - sorry if i've offended anyone, and if so, that was not my intent>