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Brass Case Trimming Part 1


Now that you’ve selected the best reloading brass cases for your shooting application, deburred the flash holes and uniformed the primer pockets, it’s time for the last step of case preparation; trimming. Trimming your previously-fired cases to proper length is a step that must be done carefully, especially when you are trying to make the most accurate, consistent loads possible.
Take too much off, and the high pressure from the primer igniting could cause severe damage. Too long and the pressure will be too low, causing low velocity and abnormal accuracy results. Knowing a little about the physics of the cartridge’s firing sequence helps understand both how a bottle-necked rifle case stretches, and why careful trimming is so important.

Case Stretch
When a bottle-necked rifle cartridge is fired, there are many events that happen within milliseconds of each other that produce extreme heat and high pressures in a very confined area. When the firing pin strikes the primer and initiates the firing sequence, it also pushes the entire cartridge forward, until the cartridge shoulder comes to rest against the chamber shoulder. The small distance left at the rear of the chamber, near the bolt face, when the cartridge is pushed forward is called headspace.
The hot gasses and pressure created by the rapidly-burning powder expands the mouth of the case as the bullet is forced down the barrel, which seals the chamber and sticks the cartridge case in place. Since the front end of the case is secured, and there’s that little bit of headspace at the rear, the same pressure and hot gasses pushing against the bullet also push back against the brass cartridge with the same amount of force, a ballistic example of Newton’s Third Law of Motion. The brass near the case head flows and allows the case to stretch a little and fill that space.
Even though case stretch happens mostly when a case is fired, it is possible to cause a little more case stretch when working the brass through your reloading dies, especially in the neck of the case. Limiting the amount of neck stretch helps you lengthen brass life and saves you money over the long run.

Why trimming is needed.
In Standard calibers, we have to trim due to the fact that as the case gets longer from the brass moving forward, the case will become too long to actually chamber properly in the firearm. Trimming allows us to bring the case back to a set measurement and give the clearance needed in the chamber. The length we trim is simply called the “trim length” Reloading brass manufacturers will often publish their recommended trim lengths in their reloading manuals and on their websites. It’s important to remember that each caliber will have a specific trim length and it may or may not be the same for each manufacturer.
How Often do you trim?
While there is no set rule on how often you are to trim your reloading brass, some reloaders tend to trim each time they reload a case. Remember, though, the more you work, or anneal, the brass, the shorter the brass will last.
Some calibers will need trimming more frequently than others. Higher pressure loads tend to stretch the reloading brass more, causing it to need trimming more often. One issue to be on the look out for when trimming is more frequent is the web section, or where the case head thins into the body. This section of the reloading brass can become too thin and cause case separation. If this occurs, discard the case. You can spot case head separation by a faint ring of discoloration around the case in the web section.
In part two of this series, we’ll look at methods of trimming, as well as tools that will help make trimming reloading brass a little easier.