I've mentioned this before, but I also use 777 and some site that I was on a couple of years ago mentioned measuring the size of the nipple to see if that was a problem with misfires. Measuring my new percussion .50, I found it to be pretty tight, Even for black powder, I believe it was .026, Which is a number 71 wire bit. So I bored it out to a 68 wire bit which is a .031 before I ever shot the rifle, And I don't have misfires. The used 45 caliber percussion I picked up last fall, which set the hammer back to safety **** On the first shot, so when I measured the hole, I found out it was a .037 or .038, Which is a way oversize. I don't load heavy, not over 45 grains of 777 which would be about 51 or 52 grains of black equivalent. The rifle hadn't been cleaned for many years, and I don't know if it was corrosion that made the nipple that big or whether it came about that size, as the rifle had not been shot very much. From what I can gather, if you load with heavy black, any time the nipple hole gets close to .032 to .035 it will start setting in the hammer back, Which is hard on hammers.
Squint