Home Forums Reloading Big Game Rifles Properly measureing cartridge headspace

Your gun will tell you when you have eliminated the gap between the case shoulder and the chamber shoulder when the case becomes tighter in your chamber and develops a crush fit. Most of the time this happens somewhere around the 3rd or 4th reloading. The headspace gauge allow you to follow the case expansion through these loadings, assign a specific dimension and check for a crush fit without having to chamber each case.

I use the Hornady Headspace Gauge which attaches to your caliper


and measures at or close to the datum line


This will give you a dimension that is relative to that specific chamber and case and can be recorded for future use. What it really lets you know is if your chamber and that brass has an excessive amount of HEAD CLEARANCE. Head clearance is the gap dimension between the point on the case that first hits the chamber when pushed forward by the firing pin and the point in the chamber that stops forward movement.

In a belted case this is more complicated because the front of the belt is the part of the case that stops forward movement. In an unbelted and unrimmed case the part of the case that stops forward movement is the case shoulder or datum line. What you have to realize is that on a belted case there is still a lot of gap, call it datum line clearance, at the shoulder even though the case has been stopped by the front of the belt.

For instance the following dimensions taken with the Hornady Headspace Gauge on cases that are neck sized only until a crush fit develops are typical:

30-06 Steyr factory
New case - 2.040" (subtracting the 2" the attachement adds)
Once fired - 2.0485"
Twice fired - 2.050"
3 times fired - 2.051" (slight crush fit)
4 times fired - 2.0515" (crush fit)
Now you can use a body die and the gauge to set the shoulder at 2.051" or 2.0505" for a slight crush fit

300 win mag Beretta Mato
New case - 2.253"
Once fired - 2.270"
Twice fired - 2.272" (slight crush fit)
3 times fired - 2.2725" (crush fit)
Time to set the shoulder back

On the 30-06 the measurement for the HEAD CLEARANCE is 2.051"-2.040"=.011". For the 300 win mag the HEAD CLEARANCE is 2.272"-2.253"=.019". The measurement of head clearance on the 300 win mag is not part of the headspace because the belted case headspace measurement is determined by the distance between the front of the belt and where it hits in the chamber. The measurement of head clearance on the 30-06 is a part of the headspace dimension and the headspace will vary directly with the amount of head clearance.

The HEAD CLEARANCE is as much a function of the brass as it is a function of the chamber. Brass manufacturers always seem to undersize the belted brass and this is why the belted cases are more prone to a case head separation.

I don't own any RCBS Precison Mic's but I think you have to buy one for each caliber. If you buy the combination set of collet for the Hornady Headspace Gauge you are set for whatever you reload for.