I suggest using " KILLING POWER" as a way to measure the relative value of various calibers and loads. Killing Power+ Velocity times bullet(ball) weight, times caliber, divided by 100. Using a .54 cal. round ball of .530, at a 100 yard velocity of 900 fps, and a ball weighing 230 grains, you get a KP of 1097.10,
Compare that to a .30-30 (.308) 150 grain bullet With a 100 yd velocity of 1930, you get a KP of 892.12. We know that the .30-30 will and has killed thousands of deer at 100 yds. It seems by comparison that the .54 is equally capable of killing a deer at that range, and from experience, we know it makes a bigger hole, and causes much more damage to the internal organs in doing so.
Note: You can use Muzzle Velocity and get larger KP numbers, but you aren't like to kill many deer at the end of your muzzle. My closest deer was about 10 feet from my barrel when I fired, but I don't expect that to happen often, or ever again. There are some tables available to give you the down range velocities of various BP loads, both Round ball and conicals. Unfortunately, they are few and far between. The latest issue of Lyman's Black Powder Handbook is probably the most complete data base. Unfortunately, even Lyman continues to give downrange energy figures in Foot Pounds of Energy, because this is the common standard for modern cartridges and bullets. The standard just leave too much lacking when discussing Black Powder velocities, round balls, and has already been noted, the killing power of balls and bullets over .50 caliber. The KP formula I have given here is a better way of comparing relative killing power of different projectiles and powder charges, at various ranges. The KP is strictly an arbitrary number, whereas the FPE number listed in most loading manuals are based on an actual measurement of energy stated in foot pounds. Originally the calculations were made using a Pendulum style impact plate. Today, formula and computers do all the work, based on muzzle velocity.
The same problem exists in measuring the killing power of large diameter revolver bullets, at long ranges. A .44 magnum revolver slug, .429, weighing 280 grains, and doing 600 fps at 200 yds has a KP of 727.27, and we know from witnessed shots that such a slug will penetrate a deer at that range, and can kill out to 600 yards, still.
So, using fpe to base a decision on choosing a hunting caliber is not a good choice of data. We do so, only because Certain game code regulations are written using that as the standard, arbitrarily, and with no data to back it up. But, those are the rules. It is up to us shooters to educate the bureaucrats to the error of their ways, and lobby for changes. Really, the folks who run the Game Departments and write the regulations welcome such comments and suggestions, because the don't like making decisions in the blind.