How to Trim the Rim Thickness of a Cartridge

When a firearm cartridge is fired, the neck expands and the length of the case increases. As a result, the neck does not hold a bullet snugly when reloaded and the case becomes too long for the chamber of the gun. Trim is an appropriate term for shortening a cartridge because the rim is ground back. With the right equipment, it is simple to trim the rim of a cartridge. The amount of time required depends on the number of cases requiring a trim.

Things You'll Need

  • Calipers or cartridge case rim gauge
  • Case trimmer
  • Allen wrench
  • Template casing
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Instructions

    • 1

      Measure the neck diameter of the case with a caliper or rim gauge. If the neck is exploded, size the neck of the cartridge prior to attempting to trim the rim. Each case trimmer pilot is designed for a specific caliber, and therefore a specific neck diameter.

    • 2

      Put the trimmer pilot in the cutter head. To do so, back the lock screw out by twisting it counterclockwise, and then push the pilot into the cutter head and re-tighten the lock screw. The head of the pilot must be flush (i.e., touch) the cutter head or the pilot over-trims the casing. Secure the cutter head in place by turning the Allen wrench screw (located on the side of the cutter head) clockwise.

    • 3

      Measure with a caliper and record the length of the template casing. A template casing is one that fits properly into the firearm's chamber. Twist the ring at the end of the chuck lever counterclockwise a half turn. Slide the butt of the casing into the chuck. Turn the lever clockwise to secure the casing in place. The rim of the casing faces the cutter.

    • 4

      Adjust the length of the trim. The two knurled adjustment rings (one large and one small) are on the cutter shaft. They are positioned on the side of the base casing opposite the cutting head side of the shaft. The rings prevent the cutter shaft from traveling too far and forcing the cutting head to over-grind the rim. Begin by twisting the large ring on the cutter shaft clockwise. The cutter shaft is engraved with length markings on the ring side of the base casing. Set the large ring on the appropriate mark and twist the small ring into the back of the large ring to secure it in place.

    • 5

      Turn the crankshaft on the case trimmer. Push on the crankshaft as you turn it to force the cutting head into contact with the rim. As the cutter head rotates it trims the rim. When the casing is trimmed to length, the knurled adjustment rings will stop the cutting shaft from advancing any farther.