Ignited by primer spark, gunpowder releases as much as 65,000 psi of gas pressure. That pressure kicks the bullet free and irons the thin front of a bottleneck rifle case to the chamber. The thick case head remains near original diameter and is thrust against the bolt face. Tightly supported, it absorbs that thrust without moving. But a “press fit” of unfired rounds allows for no manufacturing tolerances. A chambered cartridge fits loosely. Too big a gap between the bolt face and the point in the chamber that stops forward movement of the cartridge is what’s called excess headspace. Headspace is the distance from the face of a locked bolt or breech to the “datum line” in the chamber that arrests forward movement of the cartridge. 

A brass case is ductile and stretches to “take up” the tiny gap between the case head and bolt face in a rifle with correct headspace. Stretch occurs most notably just ahead of the web, where the case wall starts. Too much stretch caused by excess headspace can yield a case split, even separation. A split case spills gas that moves faster than a bullet. Unimpeded, it can cut the chamber and wreak havoc in the magazine or race back through and around the bolt to the shooter’s face.

  The term “headspace” dates to when all cases were rimmed. The measure of headspace was thus the thickness of the rim, as the rim stopped the cartridge. Rimless necked cases bear at a datum line on the shoulder; belted magnums stop on the belt. Straight rimless cartridges such as the .45 ACP headspace on the case mouth. “Go” and “no-go” gauges represent minimum and maximum measures for a given cartridge. The “go” gauge is typically .004 to .006 shorter than the corresponding “no go.”