Case Trimming

Proper trimming is always required for consistent case length. This area is often overlooked and even misunderstood, but its importance becomes evident when examining the differences between trimmed and untrimmed cases regarding loading consistency.

Traditionally, cases are trimmed when they have elongated due to repeated reloading to the extent that the length of the neck causes issues with case chambering. Allowing cases to elongate to this extent is dangerous as it can easily increase the cartridge's pressure level. Trimming should be understood as a procedure related to case uniformity rather than a necessity only when cases have elongated.

New cases are often relatively uniform, especially high-quality ones. Differences become apparent only after the first firing and subsequent resizing. At this point, the cases are closer to their final shape and dimensions, which are relevant from this point forward. During use, cases develop minor differences, most noticeable in variations in neck length.

The length of the case neck directly influences the initial stage of the combustion process. Even slight changes in the amount of neck material affect the volume of the combustion chamber, which manifests as variations in muzzle velocity. Thus, even if cartridge powder charges are measured precisely, significant variations in muzzle velocity are often attributed to differences in case neck lengths. A longer neck reduces this space, increasing pressure, while a shorter neck reduces pressure.

The Manual Specifies the Length

​Instructions regarding trimming length are typically found in various reloading manuals. However, with new cases, it's advisable to initially trim to a length where minimal trimming of the case mouth is needed. This is because, generally, cases are rarely oversized after the first firing and resizing. There may be some differences between them, but there's seldom an actual need for trimming due to excessive length after the first firing. The purpose of the first trimming is thus to standardize the cases.

​The problem of excessive shortening often doesn't manifest with low shot counts, but over time, undersized cases can cause carbon buildup in the neck of the cartridge case. This buildup increases pressure and stresses the cases, effectively reducing the size of the cartridge case neck. In other words, the cartridge case neck seems to shrink in this area. Detecting this problem typically requires using a borescope, so the user may not notice it. Therefore, trimming to extreme lengths is not advisable; it's better to stick to recommendations.

​Changes in case length mainly occur in the neck. However, in some belt-type magnum cartridges, for example, the difference between the case and the chamber is so large that stretching occurs from the first shot, particularly at the front of the base. This is often misunderstood as stretching due to the belt. Still, it's actually because the headspace in these cartridges is measured from the belt, and the initial headspace of the cartridge shoulder to the case head is inherently generous. In such cartridges, stretching after the first shot often results from stretching the case body, not just the neck.

​Neck stretching is often a part of overall case stretching, and the shape of the case plays a significant role in this regard. Typically, a shallower shoulder increases stretching, while a steeper shoulder slows it down. Modern cartridges, with shoulder angles typically around 30 degrees, are more manageable compared to older cartridge types. For example, the .30-06 Springfield, with its shallower shoulder angle, is more prone to case stretching, while in a caliber like the 6.5 Creedmoor, stretching is more moderate.

Case Trimmers

​There are several manufacturers of case trimmers, each with their own approach. The most traditional trimmers are 'lathes,' where the cartridge is secured by its base, and the neck is shortened with a cutting blade rotated by a crank. In such lathes, the method of securing the case base varies between conventional case holders and crimping collets. The issue with traditional holders is that minor variations in the case rim can significantly affect the trimming length, as the case aligns with the holder based on the rear of the case rim, where there are often minor dimensional errors. Crimping collets, on the other hand, grip the case by its edges, determining the length from the case base.

​The most rapidly evolving trimmers nowadays are those where the measurement is based on the case shoulder. These trimmers are slow to adjust but fast to use, making them particularly popular among those who load large batches and want to minimize trimming time. Typically, such trimmers, based on the cartridge case shape and shoulder, are powered by machinery, either a pillar drill or a purpose-made motorized spinner. Advanced models shorten the case mouth and create chamfers on the inside and outside of the case mouth, making them highly efficient and quick in practice.

​Using machine power instead of hand force can improve the consistency of case trimming. Machine power facilitates consistent quality because it's easier to apply the right force to the cutting blade. Since brass is a malleable and flexible material, consistent machining translates into a more uniform end product.

​After shortening the case, there are burrs on both the outer and inner sides of the case mouth. Typically, simple hand tools are used to remove these, but many overlook this step by using case trimmers with additional chamfering blades. While removing burrs is an easy task, it can be slow when loading large batches, hence the temptation to use combination blades, especially in practical shooting.

​When loading small batches, the speed of case trimming and chamfering doesn't play a significant role. Therefore, moderate trimming equipment suffices, for example, when the motivation for reloading is to produce either hunting loads suitable for precision work or enhanced performance hunting loads. It's essential to keep in mind the motives driving the reloading hobby. Many instructions, devices, and procedures are based on practices involving large volumes of reloading, so applying them directly to one's purposes may be insignificant.

​As with any stage of the reloading process, it's essential to remember one of the golden rules of reloading: all steps must be carried out, so if any step is different from your liking, simplify it. Personally, trimming isn't my favorite task, so I prefer to use machine power to assist. In this regard, a traditional trimmer with a cordless drill works perfectly.