While touring the FN factory last week, I got an up close and personal look at their process for cold hammer forging barrels. In case you don’t know how that works, they stuff a steel rod, with an oversize hole for a bore, into a machine that hammers the living snot out of the barrel blank. A mandrel inside has the mirror image relief cut of the desired rifling pattern. As the hammers mash the steel into the mandrel, the bore takes the desired shape. It’s kind of like doing auto repair with a hammer – just pound away ’til all is right.
Why do I mention this in an article about cartridge case trimming? Because those steel rod barrel blanks stretch several inches during the process of taking such a drubbing. A similar thing happens when you resize rifle brass, although the process isn’t quite as dramatic. When you stuff the cartridge case into a resizing die, it’s subject to pretty intense pressure from the outside in. That pressure reforms (basically crushes) the brass back into its original size and shape. During this process, the brass stretches in the direction of the case mouth. Ipso facto ergo e. Pluribus Unum, after one or more resizings, depending on the brass type, caliber, and load, the cartridge case is a little longer than it’s supposed to be.
Before you begin the process of adding new propellant and seating and crimping a new projectile into place, you need to trim that case back to its “trim to” length. If you don’t, you’ll get all sorts of wonky results when trying to crimp. Or maybe your newly completed cartridge won’t fit into the chamber. Or worse yet, you might end up pressure variances that impact accuracy depending on how long the case is compared to spec. For all these reasons, you’ve got to inspect the length of your bottleneck rifle (but sometimes pistol) cases and if necessary, trim them back to the proper length.
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 leaves a rough, flat cut. Chamfering applies a slight bevel to the inside of the case mouth to bullets 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 we get into trimmer types. 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.
Let’s look at a few approaches with an example of each. I’ll try to roughly order these case trimming options from least expensive and lowest production rate to pricier but faster.
It’s hard to go wrong with a classic hand-operated case trimmer like the Forster Original Case Trimmer. These are miniature lathes. One end holds the cartridge case while the other has a blade that turns via a handle to trim the case mouth. Specifics vary by manufacturer, so check to see if your model of choice (like the Forster) requires specific size collets and guides for the case base and mouth respectively.
While the trim rate is limited by how fast you turn, precision is excellent with a quality model like the Forster. Without a 75 horsepower motor, you have excellent control over even the lightest trim settings. Many models have an optional adapter to add a motor or power drill so you can ditch the hand cranking.
Most hand trimmers execute a flat cut, so you’ll need to chamfer and deburr separately. The Forster Original runs right about $100 and comes with a variety of collets and pilot guides. Other brands run a bit less depending.
Little Crow WFT Trimmers
Another nifty invention is the Little Crow Gunworks World’s Finest Trimmer. That’s WFT, not WTF! This clever device mounts in a standard hand drill or drill press. Instead of mounting the cartridge case in a lathe for trimming, you simply press a case into the unit while it’s spinning. The outer body sets length by stopping the case shoulder at a point you adjust. Each case takes seconds to trim so you can move through a pile pretty quickly. Between pressure and vibration, your fingers will get a workout, which might be a good thing if you’re learning to play the banjo.