We have all heard the term- and probably read dire warning about swapping bolts or barrels- and being sure that a smith has checked headspace on a used gun- but what IS this thing? And why should I be concerned about it? As an oversimplification of the term- it is how much free space exists BEHIND a chambered cartridge. Too little, and the bolt or slide will not close, or a cylinder turn. But when headspace is excessive, very bad things may be about to happen. When a cartridge is fired, the firing pin pushes forward on the cartridge. The rapid expansion of gasses from the burning powder causes the cartridge case to expand- gripping the walls of the chamber, and forming a gas tight seal. As pressure drops, the case "rebounds" and shrinks, allowing it to be extracted. However, between those two moments, the bolt, slide, or face of the recoil shield is supporting the head of the cartridge case, and keeping it from stretching away from the forward part of the case that is locked to the chamber walls. Some firearms can have headpace changed by repair or replacement of key parts, but some are well known for developing headspace problems due to wear and use. Perhaps best known is the SMLE- the Short Magazine Lee Enfield, and it's sucessors, the No.4 Mk1, and the No. 5 carbine. The design of the bolt and reciever, with locking lugs at the REAR of the bolt, creates a much longer section that holds headspace to designed tolerances. However, the rifle was designed so that as it wore, bolt heads could be replaced with sucessively longer heads, countering the stretching effect- IF the rifle is properly maintained. The photo below shows 2 fired .303 Enfield cartridges- a normal one, and one from a rifle whose owner was invited to clear the rifle, and remove it from the range. This was new, commercial .303 ammo. In addition to the split that goes more than halfway around, the primers were falling out of their pockets. The owner had merrily fired 10 rounds before he was stopped. In the photo you can see where the casing has been reduced in size between the rim and the crack, due to stretching the brass. This rifle was venting hot, high pressure gasses to the rear, and the owner was not injured due to the design to route gasses away from the shooter. In some cases, the violent SLAM of the case head against the bolt may shear components, blowing the bolt back at the shooter. It is a wise move to have firearms checked that are older, or had repairs to barrels, bolts, or slides- and to take a look at your fired cases for signs of problems- stretching, cracks, or primers that just don't look right.