Q: I've been given a set of three headspace gauges for the .303 British cartridge for use by our military rifle club, since most members shoot either SMLEs or P-14s. The gauges are marked Central and 064, 0.67 and 0.70. Can you tell me how they work?
A: Your set of three headspace gauges are Go, No-Go and Field. The Go gauge (0.64) measures the minimum distance between the face of the breech and the second indexing point. Ideally the gun should lock up or go into full battery on the Go gauge. If it doesn't, and the bolt won't fully close and lock, you'll know that the headspace is below generally accepted military standards. In other words, the chamber is too short. If it does close on the Go, then you insert the No-Go gauge (0.67) which measures the maximum acceptable headspace. The bolt should not lock up on the No-Go gauge. The Field Gauge (0.70)is basically just a bigger No-Go gauge. If the rifle closes on the Field gauge, you have excessive headspace and it should be checked out and corrected by a gunsmith. This is done by merely exchanging bolt heads on an SMLE, but more complicated with a P14.