Headspace is how far forward a cartridge sits in the chamber: the distance from the bolt face to whatever part of the chamber stops the case from sliding any farther. Every cartridge needs a hard stop like that, and different cartridges use different parts of the case for it.
Headspace is this distance: from the bolt face to the point where the case stops. On a .308 Winchester it's about 1.630", and a chamber is only allowed a few thousandths of an inch more. Gunsmith gauges for it run from 1.630" (GO) to 1.634" (NO-GO), a window about the thickness of a sheet of paper.
The primer gets hit rightIf the case can slide forward, the firing pin can push it away and give a weak strike or a misfire.
The case is supportedWhen it fires, the brass swells to seal the chamber. It needs to be held snug so it doesn't stretch or crack.
Every shot is the sameThe round sits in the same spot every time, which helps consistency and accuracy.
Four ways a cartridge stops
Barrel (cut-away)Bolt faceCaseWhere it stops
Stops on
The rim
Rimmed rounds
The rim catches on the back edge of the barrel. Simple and positive, which is why lever guns and revolvers use it.
Examples: .30-30, .45-70, .38 Special, .357 Mag, .22 LR
Stops on
The shoulder
Most bottleneck rifle rounds
The sloped shoulder of the case seats against the matching slope in the chamber. This is how nearly all modern rifle cartridges do it.
Examples: .308, .30-06, 6.5 Creedmoor, .270, the PRCs and WSMs
Stops on
The belt
Belted magnums
The front edge of the belt butts against a small step cut into the chamber. Only belted magnums work this way.
With no rim or shoulder to use, the front edge of the case stops on a small step at the front of the chamber.
Examples: 9mm, .40 S&W, .45 ACP, 10mm
Why the .375 H&H got a belt
Holland & Holland's 1912 magnum has a long, gently sloped shoulder that made a poor, mushy stop. The belt gave it a crisp, positive one.
Later magnums like the 7mm Rem Mag and .300 Win Mag kept the belt mostly out of tradition and marketing, even though their sharper shoulders didn't need it.
Why handloaders prefer the shoulder
With a belt holding the case, the brass can stretch forward when fired. Each time it's resized it thins just above the belt, until it eventually cracks or separates there.
Cases that stop on the shoulder stay snug and usually last more reloads. That's why newer magnums like the PRCs and WSMs dropped the belt.
⚠Headspace problems are a gunsmith job
Too much headspace lets the case stretch when it fires. It can crack or separate and send hot gas back toward your face.
Too little headspace means the bolt won't close on a round, or closes only with force.
Warning signs: a bright ring around the case just above the base, light dents on primers, misfires, or a bolt that's hard to close.
Never force a bolt closed. Gunsmiths check headspace with go/no-go gauges, especially after a new barrel or on old military surplus rifles.
Diagrams are simplified and not to scale. General guidance, not gunsmithing instructions.