Derimming is the process where the rim of the .22LR case is flattened out to make a smooth bullet jacket. The derimming process is simple and straightforward – using a dedicated derimming die on a strong reloading press, the .22LR brass is pushed through the die with a punch. Clearly, the tolerances of the die and punch are critical for this process.
Preparation for derimming includes sorting the cases by headstamp. There are two reasons for this. First, different headstamps can have different thicknesses of brass. Running the same headstamp through the die results in a similar amount of force and ‘feel’ for that headstamp. Consistency is important in order to produce consistent bullets. Also, different headstamps tend to weight different amounts, so sorting by headstamp early in the process, and doing them in batches, helps ensure consistency.
The derimming die is a push-through die – the .22LR goes into the bottom, is pushed through the die by the punch and it leaves the top of the die as a bullet jacket. Shown below, I’ve derimmed brass with my press mounted horizontally. This provides two benefits – first, the jackets simply fall into a container as they leave the die. Second, derimming takes the most force of all of the swaging steps. Mounting the press horizontally allows me to derim the brass while standing, and it provides great leverage.
Alternatively, derimming can also be done with the press mounted vertically (below). I’ve switched from the horizontal orientation to vertical orientation to avoid remounting the press for other operations. The challenge, then, becomes catching the jackets as they leave the die. There are several solutions for this, but I chose to design, print and use a purpose-built container that is similar in design to the Lee push through sizing die systems.
Importantly, each time a swaging die is used, the brass must be lubricated. A lanolin-based lube is used. Like when resizing bottleneck cartridge brass for reloading, the lubrication keeps the brass from sticking to the die as it is stretched.
Once the brass is derimmed, I wet tumble again. This serves two purposes. First, it removes all of the lube from the brass. Since the next step in jacket preparation is annealing, removing the lubrication at this stage ensures that the lube doesn’t get burned onto the jacket. Second, when the rim is unfolded, it exposes additional priming residue that may not have been cleaned out during the initial wet tumble. I understand that not all bullet swagers go through this extra step of wet tumbling, but to me it is an easy way to ensure I’m working clean, and it doesn’t take much of my time.