The right crimp for pistol cartridges
For many years, I've been trying to figure out: What's the right crimp for my .38 super calibre cartridges? I've heard a lot of different opinions and seen a wide assortment of examples among IPSC shooters.
The opinions ranged from an extremely tight roll crimp, which according to that particu-lar shooter saves a few tenths of a grain of powder, to those that discount crimping en-tirely. Everyone has an "expert opinion".
For me, the following factors are important:
1. Reliable feeding during the pistol's loading phase
2. Precision (target pattern)
3. Uniform pull-out resistance (pressure curve, speed)
To ensure reliable feeding, I press the case mouth lightly against the bullet. As I use cy-lindrical cases, the crimp can be verified visually extremely well using a steel square or a ruler. As a check, I take apart one cartridge in each lot and examine the bullet; it should be virtually unmarked.
I've found that cartridges that are crimped too tightly, so that the case opening actually cuts into the projectile, are less precise. I've also noticed that the pull-out resistance is noticeably reduced, so that sometimes I've even been able to twist out the bullets by hand.
During the RS24 test phase in summer 2013, I had the opportunity to observe a variety of tests in the shooting tunnels in Wimmis. The bullet extractor here was extremely practical, as it also permits measurement of the pull-out resistance.
Surprisingly, roll-crimped cartridges, i.e. those in which the mouth of the case bites into the bullet, showed the lowest pull-out resistance. Cartridges, in which the case opening was only lightly pressed against the bullet, so that the case holds the bullet along the entire cylindrical length, had the greatest resistance. Their speed was also the most uni-form.
For competition cartridges, it is important to always use the same type of case. Wall thicknesses can vary considerably depending on the manufacturer, which affects the crimp. My takeaway: Instead of a tight crimp, I prefer a large case mouth/bullet contact surface for achieving the necessary function.