That shortens it slightly more than the sizing die can recover it from.ĩ mm runs right on the boarder line of enough pressure to stick brass. Instead, the whole case backs up like a piston, stops at the breech and fattens to seal and fills the chamber.
In a shrinking round the pressure never gets to the level needed to stick the brass to the chamber. The casehead is too thick to stretch under normal pressure, but with the case walls stuck to the chamber, the only way the pressure can push the casehead against the breechface is by stretching the brass next to it. But because the case head is forward by the amount of the looseness in the chamber, there is an air gap between the casehead and the breech face. The primer then ignites and pressure builds to the point the brass sticks to the chamber wall. What grows a case starts when the firing pin pushes the cartridge forward by whatever amount the headspace is loose. I once tracked a batch of cases through 50 light target reloadings, and they were all about 0.020" shorter when I retired them. The case mouths just keep getting farther away from its cutter. The Lee trimmer I bought for them back before I had much experience has never been used because it has never been able to touch the brass. 45 ACP cases lose almost half a thousandth per load cylce firing target loads. If you seat a bullet deeper in, the pressure goes up.ĭepends on the load. What is critical is the distance from the base of the seated bullet to the casehead because that controls how much space the powder burns in and that, in turn, controls pressure. The case mouth will just be higher up on the bullet in some rounds and lower on others, bu the bullet nose will still be the same distance from the casehead. So, the case length doesn't affect the COL. If you think about COL, you will realize that because the reloading press ram pushes the case up from its base, the depth the bullet is pushed in on seating is with respect to the base, not the mouth of the case. For that purpose you can trim them once and if you subject them all to the same loads the same number of times, they should continue to have matching length throughout their useful life. If you are putting a roll crimp into a bullet cannelure, that is the only instance in which you might find case length critical. With jacketed bullets crimping is usually unnecessary except to remove the flare from the case mouth after the bullet is seated. Unlike a roll crimp, the purpose of the taper crimp is not so much to grasp the bullet hard as it is simply to prevent the bullet from backing up into the case under recoil. If you are applying a taper crimp, though, the case length is less critical since the crimp occurs over greater distance and so case length variation is a lower percentage of that total crimp distance. If you are roll crimping, uneven case length can affect the crimp strength significantly. Unless you have a high enough pressure cartridge to grow the case with each shot, as some heavy magnums can, you have no need to trim for the first reason. 45 ACP, they can actually shrink with each load cycle. Many pistol cartridges keep their length or, especially in lower pressure rounds like the. The second is to level the distance from the casehead so that your crimps apply consistent pressure. First is to prevent a case from getting so long that it jams the case mouth into the throat of your barrel, which can cause very high pressure, especially in a rifle with a ball throat that tapers from the freebore straight into the lead.
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