Of course a smoothbore shooting any RB is not going to be accurate at 200 yds. And, NO, its not a proper hunting distance to use with such a gun and load. Most deer are shot at under 50 yards, and quite a few at less than half that distance. After two years as a deer checker, back in the 60s, I can also tell you that even where shotguns are using shotgun slugs, more deer are killed at less than 25 yds, than at 100 yds.
The purpose of the comment was to remind you that the RB out of a .75 cal. gun weighs a lot- way more than 1 oz. and all the MASS carries very far, and is more than enough to kill any thin skinned animal( including you) at 200 yards, if you are hit. Hitting you may be a function of bad luck.
As for trajectory at 200 yards, it depends on the load used. Use that 60 grains charge of FFg in a paper cartridge, which we are told was the standard military load, and velocity at the muzzle is less than 1,000 fps. Start using those 4 and 6 dram load now being discussed above, and the velocity is going to be much higher, and the trajectory flatter- but you will probably still have to aim at his head to hit him in the feet at 200 yards. ( The faster it goes, the faster it slows!)
I can't find any trajectory information for the Brown Bess in the literature I have on hand. I do have some idea of what a 12 gauge( .729) diameter slug does out to that kind of yardage, fired from both a rifled barrel, and from a smoothbore.) Considering the larger diameter of the Brown Bess ball, look to 10 gauge loading data to get some kind of idea about its power, velocities, and trajectory. ( 10 Ga. is .775" nominally).