By Duane Thomas
While serious handloaders pride themselves on turning out ammunition so bright and shiny and well-assembled that, absent a few nicks and scratches to the cases as they cycled through an autopistol’s action, they could be mistaken for factory ammo, there are a few things that need to be understood in that regard when it comes to 9mm Parabellum: (1) That’s really not possible; if you look closely, and know you’re looking for, you can always tell 9mm handloads from factory ammo. (2) This is no big deal. Allow me to explain.
9mm Parabellum is the most popular centerfire pistol cartridge on the planet, therefore the one most frequently handloaded. It’s also the only popular pistol cartridge featuring a tapered case, i.e. its base is wider than its mouth.
While there are such things as all-steel pistol resizing dies, most sizing dies are made of carbon steel with a carbide ring insert. (Commonly called a “carbide sizing die” – I know, how did they ever come up with that?) The carbide ring is a circular piece of material considerably harder than steel, therefore carbide has less of a tendency to “grab” cases as they’re being extracted from the die. This leads to less wear and tear on the machine, and your joints, over time while operating the machine since less force is required to cycle the handle. Therefore, the machine can be operated without using case lube, which is pretty much a necessity with all-steel dies – though using case lube is still a good idea and will slick up the machine’s operation even with a carbide sizing die.
A sizing die doesn’t resize a case along its entire length. First, we have to take into account the shell plate, which blocks the sizing die from going all the way down. Then there’s the necessary taper, or flare, to the sizing die, necessary to funnel the case into the die, which means the portion of the die that actually touches the case starts a bit up inside the die. We’ll also want a bit of a gap between the bottom of the sizing die and the top of the shell plate. Not a lot, but it needs to be there.
Put it all together, while the die resizes the case walls, it’s doesn’t resize the base, that portion of the case right above the extractor groove surrounding the web, where the walls progressively thicken inside the case, curving down and inward to increase case thickness, preventing case blowout in the area where the case is unsupported over an autopistol’s feed ramp.
Also worth noting, 9mm Parabellum, since it was designed from the ground up to be a high-pressure cartridge (SAAMI maximum chamber pressure for even standard-pressure 9mm Parabellum, let alone +P, is 35,000 pounds per square inch, the same as for .357 Magnum) is a notably strong little case, with a really thick web.
While the carbide rings for most pistol resizing dies are not very tall – at their shortest they look like a miniature hula hoop, at most they’re a bit taller and look like a miniature wedding ring – the 9mm Parabellum carbide ring is a notable exception. Short, i.e. not very tall top-to-bottom, carbide rings can resize a straight-walled case since all they have to do is go straight down along the sides of the case. The 9mm Parabellum’s carbide ring is about half-an-inch tall because it has to reapply the taper.
The fact the sizing die doesn’t full-length resize, it only resizes the thin case walls, is deliberate. We don’t want to work the base hard, this could weaken the base in that area, leading to a case head separation during firing, which occurs when the rim and base rip away from the main body of the case, leaving a ring of material stuck in the chamber. The gun is out-of-action until we can get in there, typically with a pair of needle-nose pliers, and remove that ring of material.
Okay, so, we’ve resized our case back to a close approximation of its original dimensions, i.e. it’s tapered with the base of the case wider than its mouth. We slightly flare the case mouth to accept a bullet. (We’ll also be dropping a powder charge, though that doesn’t affect the shape of the case.) Then, when we seat a bullet, that causes the top part of the case to expand. But the area of the case right under the bullet’s base has not been expanded, instead it nips in around and under the bullet, providing a “shelf” preventing bullet setback when the bullet tip hits an autopistol’s feed ramp on its way into the chamber. Then we crimp the case mouth back down to get rid of its flare.
When all is said and done, since the 9mm Parabellum’s base is visually, noticeably wider than a straight-walled case relative to the top part of the case, and the top part of the case has been expanded down to the base of the seated bullet, we wind up with a cartridge that has a bit of a wasp-waisted effect where the wide base sits below, and the portion of the case that’s been expanded by having a bullet seated sits above, a resized but unexpanded portion in the middle. I call this “the Coke bottle effect” because I find the overall shape reminiscent of the classic old green-glass Coke bottles.
The important thing to realize is, the Coke bottle effect is no big deal. It has absolutely no effect on a cartridge’s safety or feeding characteristics. This is not indicative of any flaw in your reloading procedures, on in your handloads. Rather, it’s an indicator the system is working as it should.
