Here is what I HIGHLY recommend for the nipple:
http://www.dixiegunworks.com/product...49c49fd9ce6b9b
This product is a nipple with an O ring on it. Then it has a brass cover that attaches to a leather string to tie on to the gun. What you do is put the cover over the nipple after loading the gun. The O ring holds it in place and makes it waterproof. It is a totally weather proof cover, and it is a safety. Even if you have the gun loaded and a percussion cap on the nipple, with this cap in place, you can pull the trigger and all that happens is it hits the back of the cover. When ready to fire, just pull it off and shoot!
About 777. You might have some problems with ignition. I went to using fffg powder. And when I load, I pour about 15-20 gr down the barrel and bump the barrel trying to get some of the granules to fall into the side channel. Even so, when I hunt, I load up, then remove the nipple and pour a couple of grains in the channel, then put the nipple back on. Probably not totally kosher doing that, but it has always worked for me. Never had a misfire in the field.
But remember that fffg is more powerful than ffg, so reduce the load by about 10% from what you would use for ffg