Can someone explane why a fast burring power like IMR 3031 can be used in a light weight bullet like a 257 ROBERTS in 87 gr. and also be used in a 4570 shooting a 300 grain bullet?
Hab

Can someone explane why a fast burring power like IMR 3031 can be used in a light weight bullet like a 257 ROBERTS in 87 gr. and also be used in a 4570 shooting a 300 grain bullet?
Hab

It has to do with the bullet cross section, expansion ratio of the case capacity to bore, and bullet weight. The smaller lighter bullet has less mass to move, even though its cross section area is smaller and the expansion ratio of the case to bore is higher. So, as the powder burns that light bullet moves quickly and easily down the bore increasing the volume of space the expanding gas has to fill. Pressure stays within safe limits. The .45 bullet has more mass to move, but a larger cross sectional area to work against and as it traves down the bore, volume increases faster than the smaller bore for every inch of travel, so the gas never can quite fill the expanding space quickly enough to raise pressure to unsafe levels.

However, the powder will burn differently in each bore, with higher peak pressure in the smaller bore and lower in the larger bore. Powders burn at different rates in various circumstances, where pressure and temperature have an effect on speed of burn. So the .257 load may reach 50,000 psi where the .45-70 load only produces 40,000 psi even though the .45-70 load uses more powder. However, both pressures are within normal working limits for the cartridge and rifle type. This alchemy is what makes shooting and loading such an art as well as science - it's just so much more cool than golf!

axlenut

Take a hard look at a good loading manual...look at .219zipper (or .22Savage), 25-35, 30-30, and 38-55. Same base case, only the bottle neck and bullet diameter changes...but the "best" powders kind of change from one to the other.

For that matter, can look at the .250savage and it's better powder choices and compare it to the .22/250....or the 25-06 vs. the 30-06 vs. the .35Whel.