Can You Measure Rifle Headspace with a Comparator Tool?

When it comes to measuring rifle headspace, one of the most common questions we hear is:

“Can I use a comparator tool like the Forster Datum Dial to measure my rifle’s headspace?”

The short answer is: Yes… and no.

Here’s why.

Forster Headspace Gages are manufactured to precise SAAMI specifications and are marked with the exact chamber dimension on the gage itself. For example, a Forster 6.5 Creedmoor GO Gage is marked 1.5410”, representing the SAAMI minimum chamber dimension measured from the .400” datum line on the shoulder to the case head.

However, if you measure that same gage using a comparator tool such as the Forster Datum Dial, you may notice the reading differs slightly — sometimes by .002” or even more.

That’s because comparator tools are designed for comparative measurements, not absolute precision measurements. Producing them to the same tolerance levels as certified headspace gages would make them significantly more expensive. 

During Forster’s early development of the Datum Dial, tighter tolerances created edges sharp enough to mark or cut brass during measurement. By adding a slight radius to improve usability and protect brass, the contact point changed slightly — making the tool excellent for comparing cases, but less ideal for directly measuring gages.

That said, comparator tools are still extremely valuable.

While a Datum Dial may not give you a true certified headspace measurement on a gage, it can still tell you a lot about what’s happening in your rifle.

A practical way to use it is to first zero your comparator using a known headspace gage, then measure your fired or sized brass. The difference between those measurements can give you a much better understanding of your rifle’s chamber headspace.

For reloaders who want the most precise way to evaluate headspace, stepping through measurements in .001” increments can provide an even clearer picture of chamber dimensions from minimum to maximum spec.

Whether you’re checking brass consistency, setting up your sizing die, or simply learning more about your rifle, understanding how your tools are intended to be used makes all the difference.

Questions about measuring headspace or selecting the right gages for your rifle? We’re always happy to help.