im new to relaoding.... why again does the case need to be 3/4 full?
The short answer is, it doesn't.
For loading full power rifle loads, lets say bottle neck cases, the most EFFICIENT loading, is with a type of powder that fills the case. This is because the rifle will get its best velocity with as slow a burning powder, as it can handle. In other words, if a case full, or nearly so, of powder will give it maximum desired pressure, it should also give it maximum velocity.
Now, about a partial case full of powder.
Sometime after WW2, some reloader said that less than a full case of the slowest of powders, in this case H4831, blew his rifle up. After no end of theories, it was decided that somethng called detonation, instead of burning the powder, blew the gun up. There was no end of reasons why this happened. Everybody got on the bandwagon and said not to load H4831 at less than a case full.
So, Bruce Hodgdon, the originater of Hodgdon powder, went to work in their lab. With their pressure testing equipment, he tested everyone of their slower powders at less than a case full, going right down to squib loads with them. Many hundreds of rounds were fired and I have the results of the test. He was unable to prove in the lab that any type of partial load with any of their slow powders, produced anything except normal pressures. That is, heavier loads gave more pressure, while reduced loads gave proportionately less pressure.
For the last hundred years reloaders have been producing reduced loads in their firearms, using partial case loads of every powder in existance.
Now, in our world of litigation and sue happy lawyers, every chicken little that starts to reload tells every other chicken little, that he must not load less than a nearly full case of powder, or his gun will blow up! Through some word of mouth among these chicken littles (the sky is falling, the sky is falling) it is now accepted "fact," among many, that a load should not be lighter than the given starting load in a loading manual.
What a crock of BS!!!!!
I will defy anyone to show proof that any partial load of fast, or medium burning powder, has ever in history, caused dangerous pressures in the gun.