I don't see a round ball with iron sights being accurate enough to hit a man much beyond 300 yds. with any regularity. If you use a tang peep sight, and a globe front, technically still " Iron " sights, you might be able to stretch that range out another 50 yards. After that, the B.C. of the Round ball, no matter what diameter, is so poor that any wind is going to make it difficult if not impossible to hit a man sized target much further, other than by luck.
With Conical bullets, its an entirely different matter. Since you don't make it clear in your post what you are asking about, and there are flintlock ignition rifles made to shoot conicals, it would be possible to hit a man sized target at more than 500 yards with the right gun and peep sights.
Most of the large slug gun rifles are fired with an altered percussion ignition system, not flint, underhammers, not side hammers, or side lock, and they use scopes for the long range targets, beyond 500 yards. There are some Black powder cartridge guns, ala " Quigley", that have long range tang mounted peep sights, shooting lead bullets, in front of black powder, and they can hit man sized targets at 800 yards and beyond. That 1/2 mile, and that is a very long shot. Can a flintlock do the same? With the same sights, a quality barrel, well tuned lock, and the right bullet and powder load, I don't see any reason why not. However, the barrel is likely to be much larger than the barrels found on a Sharps rifle, and I think you would have to have a longer barrel to get the velocities with an open ignition like a flintlock, compared to the more closed ignition of a percussion or cartridge gun.
A friend had a Bench Slug gun that fired a 10-shot group at 500 yards that measured 5.26" in width. That gun, with its scope sight, was obviously capable of hitting a man-sized target much further, and probably out to 1,000 yards. Iron sights? Flint? The gun didn't have it, but he was shooting Black powder behind a very large bullet.
I hope that answers that question. :hatsoff: