rifle, gun-safety, military-surplus, repairI’ve seen warnings about checking headspace in discussions of military surplus rifles where the mismatched parts may have been used to build a new rifle. What is headspace and how do I check it?
Headspace is the distance from the breach face to the datum surface. The datum surface varies by caliber. On most rimless / rebated rim straight-walled brass it is the case mouth. On rimmed cartridges it is the forward edge of the rim. On a belted cartridge, it is the forward edge of the belt, and on most other necked cartridges it is a point on the shoulder.
Normally you check headspace, rather than actually measure it. (That is, you’re looking for pass / fail rather than a number). That said, headspace is checked with a set of two gauges - a ‘go’ gauge which proves that the chamber will accept a round, and a ‘no-go’ gauge which proves that the chamber will not chamber an out-of-spec round. To test, you want to remove any spring-tension, such as might be caused by the ejector in an AR-15 bolt, or the cocking mechanism on an Enfield. That done, you attempt to close the action on each of the gauges. The action should close easily on the ‘go’ gauge, but not close on the ‘no go’ gauge. The ‘no-go’ gauge is the one you normally are concerned with. If the action closes on a no-go gauge the chamber is too deep. At best this will result in overly deformed cases during firing, but it could also lead to a case rupture, which can be dangerous.
If a rifle is found to have excessive headspace (closes on the ‘no-go’ gauge) it can be ‘set back’ on most rifles by a gunsmith as long as the chamber is relatively intact. Some rifles (the AK’s come to mind) and most pistols aren’t worth the effort and would warrant a barrel replacement.
“Headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge (the datum reference) to the face of the bolt.” (from Wikipedia)
Here’s a good video about measuring headspace on a bolt-action rifle.
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