This is an arguable topic because close-range deer have been killed with almost everything that would throw a bullet out of the muzzle of a gun, including the .22 L.R. The fact that a given load has sometimes killed deer does not, of itself, mean that it could, or should, be used under all circumstances. Obviously, a survival situation is different from hunting where every effort should be made by the sportsman to deliver a projectile into the animal that will be quickly lethal.
Shot placement is more important than power. An ill-placed shot from a powerful gun will not reliably kill deer, but a well-placed bullet will. The selection of a load for a young hunter should be an accurate one that he, or she, can shoot well. Because we are discussing threshold loads, it also follows that the range needs to be short, 50 yards or less. It is also of assistance if the deer is relatively small with weights in the range of 90-120 lbs. These are good-eating-sized deer and fine for a young hunter’s first deer.
In muzzleloading the .45 is considered a medium caliber. The lightest projectile in this caliber is a patched round ball. In the typical light-weight rifles that a young hunter can handle (6-7 lbs. with 28-inch barrels) I recommend 85 grains of FFg GOEX black powder and a patched round ball for well placed shots in the heart-lung area at 50 yards and less. This load will develop about 900 ft./lbs. of muzzle energy, but at 100 yards its muzzle energy will have decreased to less than 300 ft./lbs. At 50-yards it will still be producing about 500 ft./lbs. which most consider the threshold value for taking deer-sized game.
Although once more common than .50-calibers, today it is difficult to find .45-caliber muzzleloading rifles that have suitable twists for round-ball loads. In the easier to get .50 caliber-rifles, the heavier balls are more effective using the same charge of 85-grains of FFg. Because the bullets are slightly heavier there will be more recoil. At these velocities, pure lead balls will expand on game with notably more expansion if it drives through the shoulder blade.
Muzzleloaders are nice in that there is no reason to start a shooter out with maximum loads. Initially, the load with either-sized bullet can be between 40-50 grains of FFg which will give a satisfying amount of recoil and smoke but not be punishing. For any shooter, start out with a low-power load and then increase the powder charge to the point where the shooter can tolerate it before moving up in power.
If shooting patched round balls is considered too much trouble, Buffalo Bullet’s BallEts, are available in both .45 and .50 caliber. These are hollow-based pre-lubricated bullets that weigh more than round balls, have slightly better penetration, are faster to load and are a little less messy to handle. They do not clean the bore with each loading as do the patched round balls. For best shooting with the BallEts it is necessary to wipe the bore between shots. When hunting it is not usually necessary to clean the barrel between shots because of the few shots fired, but when targeting the rifle wiping the bore between shots will keep fouling from building up in the bore to the point where you can no longer load the gun. BallEts will shoot to a different point of aim than round balls (both horizontally and vertically) and sights will often need to be adjusted.
Some traditionally patterned muzzleloaders have set triggers. These help precision shooting, but do require extra care in their use. Those on Thompson/Center Arms guns are double-set, meaning that a strong pull on the front trigger will also fire the gun should the user need to make a quick snap shot. It is also more difficult to silent-cock a gun (Pulling the hammer back while also holding the trigger back and then lowering the hammer into the full-cock notch – practice before doing this in the field.), but set triggers allow more precision and help shooters fight flinch. I prefer to start beginners out with a standard trigger.