They don't always. The only time I noticed consistent increases while chronographing handguns was with lead bullets in the 900-900+ fps range. Unburnt powder fouling and slight leading on the inside of the barrel seemed to cause slight pressure increases and higher velocities. It made me wonder here with velocities under that figure. Perhaps someone more knowledgeable can shed some light on this.I am much more of a plain shooter and an accumulator of guns than I am versed about ballistics. Why does pressure rise after a few shots?