Wednesday, May 8, 2024

Bullet Migration in 9mm Revolvers

 John Farnam has an article up at on this topic. As you may realize either from practical experience and/or application of basic mechanics, when a firearm recoils, the bullets in the cases tend to try and stay in place--i.e., not moving backward or upward with the recoil. The result is that bullets that are not sufficiently crimped into place can start to work their way out of the case--migration. In a revolver, the bullet can actually work out far enough to interfere with the turning of the cylinder. I've had it happen a couple times where the bullet actually jammed the cylinder tight enough that I needed a rubber mallet to encourage the cylinder to open. 

    It's less likely in a semi-auto, but Bond Arms' Bullpup 9, because of how it operates, has a tendency to cause the bullet to completely come out of the case when cycling unless the bullet has been sufficiently crimped. Thus, Bond Arms has created a list of commercial ammunition--both defensive and range ammo--that will work with their handgun.

    Most of the time, this is only an issue in a revolver when shooting magnum rated loads with no or little crimping. The solution is using a sufficient roll crimp at the end of the cartridge. But in most semi-autos, such as the 9x19, the cartridge actually headspaces on the rim of the cartridge, so you can't (safely) use a roll crimp. Instead, such cartridges use a taper crimp which just means that the case is tightly squeezed against the bullet to prevent bullet migration without mashing in the rim. 

    The consequence, as Farnam explains, is that because 9 mm ammo does not use a roll crimp, the bullet is more likely to migrate out upon recoil, potentially interfering with the turning of the cylinder. Accordingly, Farnam does not recommend a 9 mm revolver for self-defense, writing:

I’m still a fan of snubby revolvers and own several, but my recommendation, when you share my enthusiasm for them, is to stick with 38Spl. The bullet-jump issue still exists, to be sure, but to a much lesser degree, particularly when you’re shooting high-performance ammunition, like Cor-Bon’s 110 gr DPX or Speer Gold-Dot. Manufacturers ensure that these bullets are adequately crimped-in and are thus unlikely to migrate.

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