Troubleshooting Case Trimming
Date: Nov 23 2005
Back when I started handloading in the mid-1960s, most of the simplified, step-by-step instructions on the subject left out case trimming. I don't know why, because trimming is one of the basics of metallic loading – but I had to find out myself about it, the hard way. This happened with my first press and set of dies, from the old Herter's mail-order company. The dies were for the .243 Winchester, a case that tends to stretch more than some others when fired and resized.
The first indication something was wrong came when some handloads – and not others of the same batch – showed unmistakable signs of very high pressures: hard extraction and extremely flattened primers, often combined with an imprint of the Remington Model 700's ejector hole.
It took awhile to figure things out, mostly because my handloading library was quite limited at the time. I had been trimming cases to factory maximum overall length (OAL), but some cases not only stretch lengthwise, but the necks thicken after repeated firing. This is particularly common in cases like the .243, with short necks in front of large shoulders that aren't particularly sharp in angle.
Apparently the powder gasses massage the inside surface of the case neck hard enough to actually force some brass into the case neck. This cuts down on the amount of room around the seated bullet when a round is chambered. If the process is repeated often enough, the neck is actually forced tighter around the bullet when the bolt handle is lowered. When the powder ignites, the neck doesn't want to let go of the
bullet – and pressures rise, often drastically. This was exactly why my handloads sometimes showed signs of very high pressures, and sometimes didn't: Some cases had grown thicker necks than others.
It's easy to diagnose this problem beforehand, by attempting to slip a new bullet into the neck of a fired case. If there's any resistance at all, the necks need to be thinned. At that time I ordered a simple neck reamer for my Forster case-trimming tool, but today I would turn the outside of the neck, a more precise process that can also be used to "uniform" the neck thickness of any particular batch of cases.