It depends on the application. What are you trying to do?
Grain refers to the weight of the bullet when measured on a powder scale.
The difference between a 40 grain bullet and a 36 grain bullet lies in their weight. The 40 grain bullet is heavier, which can result in higher velocity and potentially better accuracy due to increased stability. However, the 36 grain bullet may have a flatter trajectory due to its lighter weight.
The weight of the bullet. They can range from 25 grain (Small rifles) to over 400 grain for big game.
Grain refers to the weight of the bullet when measured on a powder scale.
The 147 and 124 grain are bullet weights. The 124 grain bullet loaded to higher pressures (+P) will have a higher velocity than the standard load with a heavier 147 bullet. As far as penetration goes, I have seen tests where the 147 grain out performed the 115 grain +P but the 124 grain +P should be a good all around round.
The reference to grains means the weight of the bullet.
You must specify what you desired end result is and on what medium.
The heaviest bullet in my reloading manuals for the .380 is a 120 grain cast bullet.
The muzzle velocity of a Winchester .30-30 varies depending on the load and the bullet, but we can ball park things a bit. With a "standard" load and a 110 grain bullet, it's about 2,690 fps (feet per second). For a 130 grain bullet, about 2,500 fps. A 150 grain bullet cuts it to about 2,400 fps. With the 170 grain bullet, it drops to about 2,200 fps.
.408 Supersonic Chey-Tac round. It uses a 419-grain or 305-grain cartridge, and uses a Spitzer bullet
A .325 WSM should be your smallest, or .308 with AP tip. The Ideal bullet would be a .220 grain ACT Winchester .338. Or a .338 Lapua Magnum
The grains of a bullet, be it black powder or not , is the weight of the bullet. Lets say you have a .50 caliber rifle, and fire a 250 grain bullet at a target and hit dead center. Then fire a 300 grain bullet, that bullet will hit slightly lower on the target , but will have more force or stopping power" due to the weight. So the higher the grain the heaver the bullet.