Relation between case fill and pressure?

[SKWERLZ]

Hypothetical scenario: If we change the case fill% by just tinkering with case size WITHOUT changing anything else like seating depth, powder qty etc etc...what should happen to the pressure in chamber? WIth lower fill % will pressure go up or down? what will happen with higher fill %?

To change the fill percentage you have to change something, if the useable volume increases for a given amount of powder pressure giving a smaller fill percentage pressure is less.

If useable volume is the same and you use more of same powder to increase fill percentage pressure goes up.

 

[SeabeeKen]

Keep in mind Harry, these software programs are a prediction only and not meant to replace actual reloading data and the powder models need to be tuned to each rifle and cartridge and each case and bullet must be measured for accuracy. Also keep in mind, you are putting as much as 65,000 psi next to your face and if you are off, it could be exiting the gun. READ THE DIRECTIONS FOR EACH PROGRAM

 

[CharlieNC]

If you study the graphs from Gordon or Quick Load, you will see the maximum pressure occurs a bit after the bullet has left the case and chamber, as in 1-2 inches. This is because the powder does not explode in an instant, but burns progressively over a period of time as the bullet travels down the barrel. While the case capacity and fill are certainly important factors, they are not the defining parameters as this is a much more complex mechanism, so there is no simple rule of thumb.