Let It Rip
You'll be fine. If you do happen to get a misfire... wait a safe amount of time and recap.
As somebody else said, get magnum caps if you really think you're going to have problems.
I've got a 58 cal H&R, an early inline that shoots patched round balls and I can't recall ever having a misfire... I've got 10 lbs of goex in my stash, because it will store FOREVER, but I shoot 777 exclusively in the rifle and a bunch of cap and ball revolvers with no problems whatsoever.
In my opinion 777 is pretty good stuff.