Why does a cartridge with a smaller lighter bullet have load data of typically a bigger powder charge and the heavier the bullet gets, the powder charge can go down?

Is this because of physical space in the cartridge? Or a velocity type thing?

I'm not 100% sure but think it has more to do with chamber pressure. I'm sure Chad will be along shortly to clarify.

Yes its chamber pressure. A heavier bullet provides more resistance and will cause higher pressures with the same amount of powder.

Think of it this way. If you have a 308 Win round and compare shooting a light 130 grain bullet and a heavier 175 grain bullet. Hodgdon lists 50 grains as max for a 130 grain, and 45 grains with a 175 SMK. The 175 grain bullet has a longer bearing surface (more friction in the barrel) and has more weight pushing back. The 130 grain has a shorter bearing surface (less friction in barrel). With the same powder, the heavier bullet will have more resistance, creating more pressure. To keep pressures in check, you have to reduce the powder charge, in comparison to the 130 grain bullet.

thanks...makes total sense.

Just trying to fill my brain whats behind all this. I dont like to do something without "understanding" it.

I cant just follow a formula and get good results and be ok...Im the type that likes to always have an understanding on whats going on.

thanks again

When I first started reloading, I could not understand why a 38 Special was so much slower than a 9mm in about the same length barrels with the same weight bullet, and near same size bore. The 38 Special was a bigger cartridge than the 9mm and held about the same amount of powder. The answer is found in the pressure generated by each cartridge. The 38 Special is about a 17K psi round where the 9mm is about 33K psi, which is right at twice the pressure. The 9mm case is smaller and holds about the same amount of powder with the same bullet, therefore generates more pressure inside the case.

Another related phenomenon is that the same weight bullet fired in a "line" of cartridges will always achieve higher velocity. For example, a 150 grain bullet in the .270 Win will go faster in the .280 Rem, and still faster in the .30-06. This is because the expansion chamber drops more rapidly in the larger diameter barrel.

A 140g bullet will take up more case space than a 120g bullet, for instance, so along with what others have said, you simply couldn't get the same amount of powder in the case (140g seated deeper than 120g). This especially pertains to rifles with limited mag box lengths like most all production rifles have.

Here's an explanation from a different angle: a heavier bullet will resist being accelerated more than a lighter one, just as you'll notice it takes more effort to lift a bowling ball than it does to lift a tennis ball. Next point, [nearly] all the powder in a loaded cartridge is going to be converted to gases at some certain rate. If you release a given amount of gas behind a light bullet, pressure is relieved quickly as it builds because the bullet responds by moving very readily. A heavier bullet will resist acceleration more than a lighter one which would cause the released gases to be confined to a lesser volume behind the bullet---which means the pressure will be higher.

It even compounds to an extent. The heavier bullet over the same charge will cause the powder charge to convert to gaseous form even more quickly. There is a "tipping point" for smokeless powders where excess pressures cause conversion to gaseous form even faster, which is a regenerative process. Higher pressure > faster burning > higher pressure > faster burning. This is basically why guns blow up when excess powder charges are used in handloads.

Actually, that has little to do with it. Few rifles are loaded to 100% case capacity, even though that seems to be the "popular" trend lately.

It is all about pressures created by the weight of the bullet, and little to do with how much of the bullet you can stuff into the case.



And THAT ^^^ my friend, makes you a smart reloader. It will serve you well when manufacturing ammunition.