For straight wall rimless cases such as
the 9mm Luger, headspace is controlled by the case mouth
butting up against a corresponding ledge (chamber cartridge
stop) within the chamber of the firearm.
Headspace is important for the proper
and safe operation of a firearm. Problems occur
when the headspace is either too long or too short. This may
be the result of the firearm itself or the improperly sized
ammunition used in the firearm. If the headspace is too
short the bottom of the cartridge will protrude out of the
chamber and preventing the action from closing or causing
firing pin firing problems. If it is too long,
the case may experience excessive stretching and deformation
Pistol Chamber & Cartridge Stop
When a round is fired, pressure within
the case of the cartridge builds up and expands the case
walls outwards in all directions. The sides of the case are
pushed against the chamber walls gripping them tightly. The
rear of the case may move rearward a few thousandths of an
inch, resulting in thinning and weakening of the case walls
where the case stretched, usually just ahead of the
extractor groove, or rim. As the pressure subsides the brass
will retract and release its grip on the chamber wall
permitting extraction from the chamber. Headspace problems
in general are most commonly associated with bottle-necked
In firearms, headspace is a distance measured from
chamber cartridge stop (the ridge that
stops the forward movement of the cartridge) to the closed
Headspace length = .750"
Chamber ID = .390"
Chamber ID of a 9mm Glock 26 Pistol = .390"
9MM Luger Case OD (3 range pickups, expanded) = .3860",
9MM FMJ Bullet OD = .3565"