Muzzleloader bullets are measured in grains, or the weight of the bullets. The larger the grain, the greater the force it will exert on impact. This is true in a sense. However, the larger the grain, the heavier the bullet is, which means slower speed and reduced distances. Choosing the right weight of bullet is based on what kind of game you are hunting, and at what distances you will be shooting.
Here are some basic fundamentals that I follow when recommending muzzleloader bullets to people:
If you are hunting for deer, I always recommend either a 245 or 250 grain bullet as that should give you good knockdown power and good accuracy out to 150 to 175 yards. Now if you know you are going to be hunting deer out of a stand where the longest shot is under 75 yards, I would recommend moving up the grain weight from 275 to 300 grains. This is because at that short of a distance, the aerodynamics of the bullet will not change that much, and you will have a greater kinetic force upon impact, thus greater knockdown power.
If you are going after larger game like elk or bear, I would definitely recommend being around the 300 grain mark. With these animals you want greater knockdown power, and with only one shot, you need to do as much damage as possible. I will say I have friends that have taken elk with 245 grain rounds, so it is possible to do, and you could have success with it too. For me personally, I want to be over-prepared. I would rather use a larger 300 grain round and not short-change myself.
Overall, the most popular rounds for deer (whitetail, muley, blacktail) are 245-250 grain bullets or sabots. The most popular rounds for hunting elk are the 275-300 grain bullets or sabots. I hope this information is helpful, and best of luck on your hunts!