Found this trying to get data for my .30-40 using RL-15, Is it true? Could I use info from a 220 gr. bullet on a 180 gr. bullet? Sorry for all the questions but Im new to this and cant find specific data for any of my calibers using RL-15. Ive found data for .30-30, .30-06, .303 British, 8mm Mauser, etc.... But none for .30-40 or 7.7. I have around 4 lbs. of Rl-15 and I dont understand why I have to buy a new powder when its used in so many similar calibers? Bullet Weight not Found in Manuals Lee: If the bullet weight you wish to load is not found in your favorite manual, it is safe to use loads listed for a heavier bullet weight and find a load for the powder you have. Using that load with the lighter bullet simply reduces the pressure, but the velocity remains approximately the same. Speer: The physics of loading cartridges indicates that a heavier bullet will build pressures faster than a lighter bullet owing to its mass. The greater mass of the heavier bullet resists change (acceleration) more than a lighter mass so the powder charges for the heavier bullet will nearly always be lower than those for the lighter bullet of the same construction. This indicates that, without other data to follow, the heavier bullet data can be used as a starting point for the lighter bullet.