Forster Datum Dial: One Tool for Case Sizing and Bullet Seating Checks

When you’re working on precision reloading, there are some measurements that are straightforward, and some that take some extra care and tools. When you’re measuring Cartridge Overall Length (COL) for example, you can use a set of digital calipers and just measure the length of the cartridge from base to bullet tip. But when you want to measure base-to-ogive (bullet seating depth with a focus on distance-to-lands) or base to datum (cartridge headspace), special tools are required. The Forster Datum Dial is a single tool that’s used with digital calipers that can handle both base-to-ogive, and base-to-datum measurements.

What’s Included with the Datum Dial Kit

The Datum Dial system is available in a few configurations (see the Midsouth Collection HERE for more information). The kit pictured here is Forster # DDKIT, and includes everything you’ll need to perform most base-to-datum and base-to-ogive measurements:

  • Datum Dial tool body
  • Standard Base-To-Datum wheel
  • Base-To-Ogive wheel #1 (common sizes)
  • Base-To-Ogive wheel #1 (less common sizes)
  • Allen keys
  • Instruction sheet
  • Quick reference card (with datum diameters for common cartridges)
  • Plastic storage case (with room for a second Datum Dial assembly)

Base-To-Ogive with the Datum Dial

Measuring Base-To-Ogive with the Forster Datum Dial is easy! Here’s all you need to do:

  • Attach the Datum Dial body (with appropriate bullet datum wheel installed, see chart below) to your caliper set with the thumbscrew (I like to hold the jaws shut against the tool while tightening)
  • Index the dial to the appropriate diameter for the bullet you are testing (marked by caliber/diameter)
  • Insert the cartridge bullet first into the dial, hold the base/rim end squarely against opposite jaw
  • Close the calipers gently while slightly rocking/spinning the cartridge (for settle-in, VERY slight movement)
  • Take caliper reading

Here are the dials that are included with the Datum Dial Kit, Forster # DDKIT. From the Forster Product Page:

Base-To-Datum with the Datum Dial

Base-To-Datum is a reference dimension and corresponding measurement that will tell you “in absolute terms” if your case shoulder is set back correctly. This is important for both reliability, and for accuracy. This dimension is closely related to the “headspace” dimension for your firearm. From an article I previously published here on Ultimate Reloader using 308 Winchester as an example:

With the Datum Dial, you can take Base-To-Datum measurements as follows:

  • Attach the Datum Dial body (with cartridge datum wheel installed) to your caliper set with the thumbscrew (I like to hold the jaws shut against the tool while tightening)
  • Index the dial to the appropriate datum diameter for the cartridge you are testing (marked by diameter on wheel)
  • Insert the case into the dial, hold the base/rim end squarely against opposite jaw
  • Close the calipers gently while slightly rocking/spinning the case (for settle-in, VERY slight movement)
  • Take caliper reading

If you are checking for sizing die setup and only need “relative” measurements, do the following:

  • Perform the steps above for a fired case, and zero the calipers while taking the Base-To-Datum reading
  • Size the case
  • Take a Base-To-Datum reading with the sized case: the indicated value is how far back the shoulder was pushed

The standard Datum wheel has most of the common datum diameters you’ll need. From the Forster Product Page:

Get the Datum Dial at Midsouth Shooters Supply

The Forster Datum Dial is a great tool, and it combines what normally would be multiple caliper attachments into a single system. If you want to get your own Forster Datum Dial, please check out the full selection of Datum Dial kits and products at Midsouth Shooters Supply, click HERE:

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Thanks,
Gavin

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