Rifle Case Trimming

First: If all you shoot is a pistol you don’t need to worry about trimming cases.  I don’t know anyone who trims 45ACP cases, for example.  Rifle on the other hand is a different story.

When the cartridge fires, the casing momentarily expands radially (it gets bigger in diameter) until the walls of the casing press against the rife chamber.  Almost immediately the case relaxes and shrinks back down but it doesn’t quite return to its original diameter.

During reloading, the full-length resizing die performs three functions:

1. Resetting the case diameter to its nominal dimension.
2. Resetting the rim-to-shoulder dimension to its nominal dimension
3. Resetting the neck and belling it slightly to accept the new bullet.

So, when the cartridge was fired it stretched. The sizing process returns it to its original size but that ‘extra’ metal has to go somewhere.  The answer is that the neck gets longer. After a few firings, the neck has gotten long enough that the case exceeds the maximum allowable dimension.  Trimming simply trims the case neck enough to return the case overall length to its nominal dimension.

To find the proper “trim to” length, just check your reloading manual. It’ll have that exact number so you can cut with confidence.

There are a number of ways to do this, and like most things in reloading, your choice of which one largely depends on your anticipated volume of work, your need for precision, and your patience level. In other words, you can spend more money on equipment and get more automation (and more production per hour) or you can do things by hand at a slower pace, but spend a lot less money on case trimming gear.

Depending on the type of trimmer you choose, you might also need a separate tool to chamfer and deburr the case mouths. The trimming operation often leaves a rough, flat cut. Chamfering applies a slight bevel to the inside of the case mouth to allow the bullets to seat smoothly and evenly. Deburring takes the rough edges off the outside of the case and usually applies a bevel there too.

One more thing before you look into which trimmer type is best for you. A case gauge is an indispensable tool, especially for bottleneck rifle cartridges. This gauge replicates the inside of a chamber for a specific caliber. Simply by dropping your cartridge case into the gauge you can tell if the case is resized properly and if the head space is correct to spec. It’s the best way to make sure your resizing die is set correctly. It’s also a super quick way to tell if your cases need trimming. Since you’re looking by eye, you won’t have 1/1,000th of an inch precision, but you can easily spot cases that are excessively long or short. If you really want to see what’s going on with your cases, invest in Sheridan Slotted Case Gauges. They’re significantly more expensive, but give you a view of exactly how your cartridge cases conform to a perfectly sized chamber.

Sure, this often adds an additional step into an already meticulous process. However, you will find that you can gain more uses out of a single casing, as well as produce more accurate results by getting the optimum overall case length. Plus, with the right trimmer set up, you don’t have to be a metal working specialist, either. Most allow you to preset the length and operate consistently.