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Micron Precision

Micron Precision Series - Expander Mandrel Body, .3050"

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Micron Expander Mandrel

Micron Expander Mandrel

The Micron Series Expander Mandrel was designed as an add-on part for Micron Series Sizing Dies. The expander has three parts: Bushing Retainer Cap Assembly, Expander Mandrel Body, and Expander Mandrel De-capping Pin. The system allows the user to pick the parts needed for their cartridge and desired neck tension. With the proper combination, one single die will de-cap, body-size, neck-size and expand in one stroke of the press. This eliminates the extra time of having to expand your case in a separate die. When expanding on the up-stroke while still aligned in the die body and bushing, the mandrel follows a straighter path, expanding the neck. The expander uses a floating design that allows the mandrel to follow the neck, preventing added runout.

Key Benefits

  • One operation, de-capping, full body-sizing, neck-sizing, and expanding
  • Radial floating expander design
  • Half-thousand expanding increments
  • Nitrided expander to prevent scratching or galling
  • Expander will work with different length cartridges by switching de-capping pin lengths
  • Easy, 'no tools required' setup

Why Use An Expander?

Expanders are an easy way to mitigate inconsistencies in neck tension. By using an expander to pull the brass back out after neck sizing, the uneven part of the neck is forced to the outside, which won't affect tension as much. Expanders give hand loaders who prefer to avoid turning necks the ability to achieve more consistent neck tension. Consistent neck tension has long been known to improve accuracy and, to some extent, lower extreme spreads in hand loads. Some top F-Class shooters have started turning necks and expanding multiple times to even out neck tension!

Choosing the Right Expander

All three expander parts are required, so first, pick the expander top that fits the caliber needed. Next, select the de-capping pin length for the case using the chart below. Finally, choose the expander body size based on the desired neck tension.


The typical neck tension range for a 6mm cartridge is 0.002" - 0.004" below the bullet's diameter. Assuming the bullet diameter measures 0.243", to achieve 0.002" of neck tension, 0.241" would be the expander to choose.

Case neck spring back after expanding can vary depending on neck thickness, brass brand, and number of firings. In most cases, the spring back will be less than 0.0005", but it is a good thing to keep in mind. One thing to also remember is the bushing used to size the neck. For this aspect, the less the bushing sizes, the better, so avoid neck sizing more than needed. This will increase consistency and lower runout since the brass isn't being overworked. A quick formula you can follow is:

(average neck thickness _ 2) + bullet diameter = ODLR

This will give the approximate diameter of the loaded round. Pick a bushing that will size the neck lower than the mandrel by 0.0005" - 0.001" under the mandrel size for desired neck tension. The neck will then be tight enough when it passes the mandrel to allow it to expand the neck.

Choosing the Correct Pin Length

Die Caliber Required Pin Length
6MM BR 20mm
6MM BRA 20mm
6MM PPC 20mm
6MM GT 26mm
6.5MM X 47 29mm
6MM X 47 29mm
300 RSAUM 32mm
308 WIN 33mm
6.5MM PRC 34mm
6.5MM RSAUM 34mm
7-6.5MM PRC 34mm
300 WSM 35mm
284 WIN 37mm
7MM PRC 40mm
300 NORMA 45mm
300 PRC 47mm
28 NOSLER 48mm
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Additional Information

.3050";Material=Stainless Steel"
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