Trimming Handgun Cases
Date: Jan 01 2011
The few times I trimmed straight-walled handgun cases it was a big pain in the neck. I finally convinced myself it was unnecessary and quit doing it. But recently I was loading .357 magnums and noticed, by the varying pressure on the press hand, that the cartridges were receiving different degrees of case-mouth crimp on the bullets. An inspection of the crimps showed some cartridges had received next to no crimp. Others were crimped so much the cases were on the verge of bulging. Cartridge length also varied quite a bit.
So I set to work trimming a batch of .357s and cases for two other handgun cartridges I shoot a lot – the .44 magnum and .45 ACP. Trimming the cases to a uniform length after they were sized helped produce cartridges with even crimps and loaded lengths. All that work also created cartridges with somewhat better accuracy.
Straight-walled handgun cases are different than bottlenecked rifle cases. Rifle cases need periodic trimming to keep their necks from stretching so much that the case mouth rim becomes jammed between the bullet and the chamber throat so solidly the neck cannot expand to release the bullet – and pressure goes through the roof, and maybe parts of the rifle.
A straight-walled handgun cartridge that is over the maximum length would do the same thing, but I have rarely seen a straight-walled case that is over maximum length. Unlike bottlenecked cartridges, these straight cases do not lengthen much, if any. Instead of expanding and lengthening on firing, like a bottlenecked cartridge, the straight cases expand in diameter and actually become shorter after they are fired.
I measured the length of five .357 magnum cases that had been trimmed to 1.280 inches after they were sized and five untrimmed .357 cases that had lengths of nearly a maximum length of 1.290 inches after they had been sized. After firing, the cases measured:
I also measured five .44 magnum cartridges that had been sized and then trimmed to 1.275 inches and five cases that had been sized and had lengths of nearly a maximum of 1.285 inches. After firing they measured:
In the .45 ACP, trimmed and untrimmed cases after firing measured:
A couple of those sized and untrimmed .45 Auto cases were right at the maximum length before they were fired, but they did not show any signs of high
pressure or present any chambering problems.
All this measuring shows trimming handgun cases is really not a safety step like it is with bottlenecked cartridges.
Cases of a uniform length provided quite a few benefits in return for the drudgery of trimming. Right off the bat, cases of even length were given the same amount of flare when run through an expander die. That ensured cases had enough flare on the mouths so lead bullets entering the case mouths did not have any lead shaved off the base, or jacketed bullets did not crumple the case when seated. Just the right amount of expansion of the case also lessens working of the brass that eventually causes splits on the mouth rims.
A uniform crimp imparts the same amount of bullet pull on firing, but how much crimp is required on revolver cartridges? A firm grip of the case mouth on a bullet is supposed to check bullet movement for just a moment to build pressure so a magnum case full of slow burning powder burns uniformly and completely. Even light amounts of fast-burning powders in large cases supposedly require a tight roll crimp to prevent squib loads. Most importantly, though, is that a bullet without the proper amount of crimp may unseat due to repeated recoil when other cartridges in the cylinder are fired. A bullet that is pulled far enough out of the case may protrude out of the face of the cylinder, preventing it from rotating.