I don't know anything about 777, except its not Black Powder, its more expensive, harder to ignite, and burns hotter. I do know that the .50-70- 550 cartridge killed about everything that walked or crawled on this continent from the mid-1860s, to the early 1870s, when the military adopted the .45-70-500 cartridge. Seems to me that until you talk to the powder maker, and the gun maker, about their recommendations, the very least you should be considering is reducing that load to 70 grains. :thumbsup:
Have you done any penetration tests with that load? That will answer your questions about whether you can use it on a Wild Boar- I am assuming that you have no intention of shooting a boar that weights less than 400 lbs. with that load! :shocked2:
The .45-90 Express cartridge fired a .330 grain bullet in it.( The .50-90 also fired a 350 grain bullet, using black powder.) Now, here you are with a gun that is NOT designed to be as strong as those cartridge guns, using a heavier charge in it. And you don't think there is going to be a price to pay for using such loads as a steady diet????
Your friend has properly told you that the first sign that the gun is saying "NO MORE" will probably be when it sends that nipple back over your head. At the very least, you will find the orifice burned open, and your hammer will be kicking back to half-cock. The spent cap will go flying- possibly into your forehead, and your accuracy will drop when that happens. Exactly when that happens depends on the condition of the individual gun.
Good luck to you. Just, Please, don't be standing next to me on a firing line when you fire that load. :thumbsup: