Red hawk, Andy said he was getting MOA at 400 yards.
Now I also may have mistakenly assumed that his in-line was a 45 or 50 caliber. It would certainly be more likely to be able to hold such groups with a 200 grain bullet in perhaps a 375 or 40 caliber rifle., but then again, there would be no need for so much powder. (200 grains.)
Such light bullets, as a 50- caliber 200 grain bullet are simply too light to buck air resistance efficiently. Just before and during the civil war there were huge advances in long range shooting, rifling and ballistics. Experimenters all over the western world were tinkering with methods to reliably shoot muzzleloaders out to 1,000 yds. For 140 years, the only thing that really worked was a long heavy bullet and a moderate powder charge. They even experimented with placing the percussion nipple where it would ignite the middle of the charge column to try to get more oomph out of a charge.
Even assuming that some fancy breech plug can cause detonation of the powder, that only increases the muzzle velocity. After the bullet leaves the barrel, everything the old timers discovered about ballistics remains exactly the same. Light bullets lose their energy faster than heavier bullets due to air resistance. A slowing bullet starts to yawl and pitch losing stability. At a given velocity, bullet shape, and bullet weight, the laws of physics remain the same, whether fired from a side lock, in-line or cartridge gun.
I have seen guys shoot clover leafs at 300 yards with underhammer bench guns that had 48 inch barrels and weighed 70 lbs. They have been doing it for 50 years up at Blue Mountain Muzzleloader Club near Shartlesville PA.
I have no doubt that MOA can be accomplished with a round ball at 100 yards because I have done it, with open sights. (When my eyes were better and they were 5 shot groups.) Harry Pope considered ten shots as a group.
Can MOA be done at 400 yards with a .50 200 grain bullet, odds are yes, with an incredibly syzygean stroke of luck of ageless proportions. Can it be done reliably, day after day, shot after shot, hardly.