From LongBeef on Patreon:
"What exactly is headspace? And how important is headspace in old milsurp guns vs. more modern guns?"

Headspace is basically the amount of play a cartridge has in a chamber. There has to be some to account for manufacturing variations in guns and ammunition, but too much or too little can cause problems. Too little can cause extraction problems and poor accuracy. Too much can cause poor accuracy as well, and also shoulder separations, case head separations, and kabooms.