Originally Posted by Alleykat
Of course, you don't trim pistol brass...it shrinks.
Not all of it does. Growth is caused when the firing pin pushes the case forward and then, when it fires, there is enough pressure to stick the case wall to the chamber wall against the pressure pushing back on the casehead. The result is the pressure ring area stretches so the casehead can get back against the breech. A sort of rule of thumb I've seen is that this phenomenon starts to show up in cartridges loaded to peak pressures of around 25,000-30,000 PSI and up. The actual number will vary with the chambering. I've seen .38 Super cases grow, for example.
Target loads and lower pressure cartridges tend to shrink because the whole cartridge is pushed rearward when pressure is too low to stick it to the chamber wall. At the same time it backs up it expands to plug the gas leak the rear of the chamber would otherwise be. That makes it fatter and shorter. When you go to resize it, the brass is moved rearward as well as inward, causing the loss. I once tracked 1000 W-W .45 ACP cases through 50 reloadings with 3.8 grains of bullseye under a cast 185 grain bullet. A light load. The cases lost an average of half a thousandth per reloading, and were 0.025" shorter when I retired them.