Before Thompson/Center introduced the .45 caliber & .50 caliber Hawken rifles to the U.S. market, along with packages of ready-to-shoot Maxi-Ball projectiles in both calibers; anyone that got involved with muzzleloading automatically knew that casting their own balls/bullets was going to be a mandatory part of the sport.
Fast forward to the present day, and there has been a complete paradigm shift as far as ready-to-shoot projectiles is concerned.
Swaged balls are available in dozens of calibers.
Cast & sized lead conicals are available from multiple manufacturers.
Plastic sabots, primarily available in .50 caliber for shooting. 45 caliber bullets dominate the market as the .50 caliber inline rifle has become the standard, at least here in the United States. There are literally hundreds of different factory .45 caliber bullets to choose from. Ranging from jacketed pistol bullets, all the way to monolithic brass & copper bullets designed specifically for long distance, inline, muzzleloading rifles.
One can shoot a bullet weighing as little as 150 grains, all the way past 600 grains, in a .50 caliber rifle, assuming that the rate of twist is compatible with the weight bullet one wants to shoot. And, that the shooter is willing to deal with the accompanying recoil that heavy bullets bring to the table.
Since the vast majority of big game animals shot in the United States are killed at distances under 60 yards, the muzzleloading caliber chosen only makes a difference if the hunter desires to utilize his muzzleloading rifle in the exact same manner as he would a centerfire hunting rifle.
If a muzzleloading hunter wants the ability to kill big game animals out at distances up to, say 250-400 yards, then a high performance, high velocity, muzzleloading bullet is going to be a necessity. Along with a very good piece of glass sitting atop the action/barrel.
It goes without saying, that having 100% confidence in one's ability to make such shots, is a requirement. Sufficient practice to obtain such confidence, and continued practice to maintain one's confidence is mandatory.
This is where the choice of caliber starts to leave the realm of the practical, and moves into the philosophical.
For myself, and I am just speaking for myself here, I do not own my muzzleloading rifles & pistols so that I can mimic my abilities with the centerfire rifles & pistols that I have shot over the past 50 years.
I chose to get involved with muzzleloading because of a love of early American history, particularly the longhunters.
As a result, my rifles thus far have been 3 different flintlock longrifles, one .45 caliber, one .50 caliber, and one .62 caliber. All three had barrels measuring at least 40" long. Weights were between 7 pounds, and 9.5 pounds.
All three locks were large Siler flintlocks, assembled and tuned by each of the three builders. All three flintlocks had very fast, and totally reliable ignition. With proper precautions taken, nothing short of a hard rain would cause any of the three locks to misfire.
My loading procedures, which included making sure that the flint was always sharp, wiping the face/edge of the flint before every shot, and using a copper wire vent pick to pick/loosen the main powder charge in the barrel through the touch hole, every single time just prior to priming the locks pan; ensured that hangfires/slow ignition were so rare as to be virtually nonexistent.
In reality, the .45 caliber longrifle was capable, with a 0.445" diameter patched ball loaded over 70 grains of fffg black powder, of killing any big game animal that I ever intended to hunt out to a range of 100 yards. Could I take a quartering shot through the front shoulder blade of a whitetail deer, and expect that 132.1 grain pure lead ball to penetrate into the vitals for a 100% killing shot? Absolutely not!!! But, broadside shots at 100 yards, and expect a 100% killing shot? Absolutely yes!!!
The .50 caliber longrifle was more capable than the. 45 caliber longrifle. The .62 caliber longrifle, with a 0.615" diameter patched ball weighing 348.7 grains, was/is capable of killing any big game animal in North America, from any angle, out to 100 yards, with complete confidence.
If I was going to choose a modern muzzleloading inline rifle as my primary hunting weapon, then I would want it to be equipped with a 4140CM steel, hard black nitride finished, .45 caliber barrel, with 0.004" deep square bottom grooves and a 1:14" rate of twist.
The 1:14 rate of twist, which many members here will scoff at, and say is too fast of a twist; will stabilize bore riding lead conicals weighing in excess of 600 grains, should one ever wish to shoot such heavy bullets.
This 1:14" twist will also stabilize the heaviest .40 caliber copper jacketed bullets from manufacturers such as Pittman, and others. Most of these bullets weigh approximately 300 grains, and measure almost 1.5" long. The Pittman Accumax 303 grain .40 caliber bullet measures 1.495" long, and has both a sectional density, and a ballistic coefficient that is off the charts, compared to. 45 caliber bullets.
All that is needed is for a manufacturer to step up to the plate with a .45 caliber, high velocity, muzzleloading sabot that will accept the longest of the. 40 caliber muzzleloading bullets. The current light blue .45 caliber sabot for .40 caliber bullets from Harvester will only accept bullets that max out at approximately 1.340" long.
You learn real fast that adequate eye relief is a must for the hard kickers.
