The Forster Datum Dial kit includes the tool to measure cartridge headspace and also interchangeable dials for bullet measurements.
The tool body clamps to a digital caliper. The Cartridge headspace dial (black) consists of 5 different sized holes to cover which will allow relative measurements of most regular cartridge cases. Rimless cartridge cases headspace measurements are taken from the Datum line approx half way on the shoulder to the bottom of the case head. By selecting the appropriate size hole on the dial which rotates and clicks into place you can obtain a ”relative measurement” of this headspace from a cartridge fired in your gun. Then by careful adjustment of your FL (full length) resizing die or a Bushing Shoulder Bump die you can achieve the precise ”shoulder bump” for uniformity and safety.
The two bullet ogive inserts (gold) fit into the tool to measure the relative ”ogive to base” of various bullets and it can also be used to measure bullet ogive to case head of loaded cartridges. two dial cover calibres from 17 to 338.
These type of tools are very helpful and often essential for the ammunition reloader.
When using any tool to takes a relative measure (relative measure is a measurement relative to the another) of cartridge headspace you need to ensure the case is square in the tool and also onto the caliper jaws. Use a straight edge to check the primer is actually under flush as many fired primers have a ”crater” around the firing pin hit area and therefore this can give a false reading if this prevents the case head being square on the caliper jaws.
Another way you can check is to hold the tool up to the light with a case inserted and ensure the actual head of the cartridge case sits square against the caliper jaw. If you can see light between the jaw and cartridge head it has something protruding like a cratered primer or possibly the case has not been ”squared” properly. This system gives you a good visual demonstration.
If required remove the spent primer with a appropriate depriming tool.
Cartridge relative headspace measurements are used to allow you to know precisielyhow much shoulder push the setup of the die is giving. Correct setup of the reloading die to give minimal shoulder bump is one of the most difficult operations in handloading. The use of measuring tools is highly dependent on the skill of the operator. Use a consistent firm touch. This often takes practice and possibly coaching from an experienced machinist. Always ensure your measuring tools are kept clean and in good order. Also check their calibration. We often find that many cheap digital calipers do not measure what the readout actually says. If you place a good brand name 30 calibre bullet and measure between the jaws on the body of that bullet (not the pressure ring at the heel) then the calipers should read 0.308″ or 7.62mm. Whilst this is not a definitive calibration test it is often a guide as we hear of units being significantly wrong in even this simple measurement. people tend to believe the numbers of a digital readout as ”gospel”