Case Trimming | Top Rated Supplier of Firearm Reloading Equipment, Supplies, and Tools - Colt


Case Trimming

Case Trimming
Now that you’ve selected the best cases, 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. 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. Redding Type S dies, (#749-006-568) both Full-Length and Neck Sizers, have interchangeable neck bushings, and let you removed the expander ball from the die to minimize case neck stretching. Limiting the amount of neck stretch helps you lengthen brass life, and saves you money over the long run.

Collet Trimmers

There are many options for trimming a fired case back to original specs. Collet trimmers have an insert or collet specifically sized to hold the head of a case, and some mechanism that turns a cutting blade against the case mouth. There are both hand-turned collet trimmers and powered collet trimmers. Collet trimmers are very simple to use quickly – the reason most reloaders start with collet trimmers, and many experienced reloaders continue to use them.
As common as collet trimmers are, there are unique variations on the basic style. The Hornady Cam-Lock Case Trimmer (#749-006-945)has a unique cam system that holds cases in the trimmer.  It is micro-adjustable with a fine-adjustment knob on the cutter shaft. The Redding case trimmer (#749-007-299)is a unique reverse design, in that it the cutting blade remains motionless while the case is turned against it.

Case Datum Line Trimmers

The Little Crow Gunworks “World’s Finest Trimmer” (#749-013-292) is designed to be chucked into a drill, and automatically quits trimming when the case shoulder contacts a pre-set stop. It’s easy to trim several cases a minute with one of these. Each World’s Finest Trimmer is made to trim a specific family of cartridges that all share the same datum line, so the 308 trimmer will work on 7mm-08, 260 Remington, 243 Winchester, 338 Federal, and others.

Three-Way Trimmers

Three-way trimmers are a lot like collet trimmers that you can add specialized accessories to, allowing you to simultaneously trim, chamfer and deburr cases. Forster (#749-006-472) and RCBS (#749-007-647) both offer these types of trimmers. They also accept attachments for turning case necks, clean primer pockets, and even cut hollow points into lead bullets.

Our Customer’s Favorite Trimmer

The number-one trimmer our customers ask for is the Sinclair Ultimate Trimmer (#749-101-020). It holds the case by its body taper, not with a collet. A cartridge case holder specific to a family of cartridges secures the case and eliminates any collet or pilot influence. It has a rail system that keeps all the parts perfectly-aligned, and micrometer adjustments that help make for the most repeatable, consistent and accurate trimming possible.

Deburr & Chamfer

No matter what tool you use to trim the cases, you will need to remove any burrs left in the metal and slightly bevel the edge of the case mouth, called chamfering. There are many kinds of tools designed to deburr cases. The standard tools (#749-002-551) deburr at 45 degrees, but tools optimized for use with boat tail or VLD bullets usually deburr (#749-004-113) at 28 or 30 degrees. There are hand deburring tools, or powered deburring tools for high-volume case prep. The Sinclair/Wilson Deburring Tool package (#749-004-354) includes the Wilson deburring tool and the Sinclair power adapter to chamfer/deburr cases quickly with a drill or power screwdriver.

Once you’ve chamfered and deburred the cases, you’ve completed the case preparation process. Now, you are ready to begin assembling a quality handload that will give a high level of accuracy and precision in your rifle. The next article in this series will examine the different types of case lubes available that help your carefully-prepped cases work correctly inside reloading dies.

Roy Hill
Brownells Copywriter