Powder PSI

[Riverpigusmc]

Originally Posted by Gatorade

Most loads aren't compressed. Usually they will have an abbreviation CL or something to indicate it is compressed as compression can increase the PSI quickly and dangerously. Some types of powder may take up more space in the case but very few actually squeeze the bullet down far enough to compress the powder. When creating a compressed load you fill the powder according to the weight of the charge which will fill above where the bottom of the bullet would seat. Then the projectile being seated will compress the load when seated to the OAL.

^this. Pressure in a firearm is measured in CUP (copper units of pressure). Some powders are meant to be compressed, most are not. Some powders are position sensitive (i.e. if they lay along the bottom of the case, ignition will suffer). Some powders characteristics are such that free space causes pressure spikes. In that case, OAL is reduced, or fillers are used. That is why loading manuals always list minimum and maximum overall length for each bullet and powder combination