By Duane Thomas
As you’re getting into handloading, you might look down inside a primer and notice it kind of looks like a radiation symbol down there, a trefoil with three triangular holes in it, and wonder “Why three holes?” The truth is it hasn’t invariably been that way.
The portion of the primer that fits down inside it is the anvil. So-called because its top (as the primer is laying open-side-down) is a wide, circular and flat striking surface. In-between the cup and the flat portion of the anvil, we have priming compound, which is a shock-sensitive material – in other words, you hit it and it burns. When a firing pin hits the primer cup, that mashes the priming compound between the cup and the anvil, the priming compound ignites, flame pours through the holes in the anvil, through the cartridge casing’s flash hole, and into the interior of the casing, igniting the gunpowder there. Thus flame flows out of the primer through gaps between its “legs.”
The primers used in ammunition produced in the United States were invented by a Brit artillery officer, Edward M. Boxer. (In case you’re wondering – I know I was – the “M” in “Edward M. Boxer” stands for “Mounier.”) Some early Boxer primers actually had two legs.
Now, I want to make this very clear: I’m not saying that Boxer’s early design featured two legs, and over time that evolved into three legs. No. That’s not it. Boxer’s initial design, put forth in his original patent (British patent No. 2,655 registered on October 13, 1866) had three legs from the get-go. But there was a time when certain ammo companies did produce a modification with two legs.
Early Winchester small pistol primers from the late 19th and early 20th Centuries (pre-1940s) sometimes had two-leg anvils, but ONLY in low-pressure cartridges, for instance black powder jobbies from the 1870s and 1880s like .32 S&W, .38 S&W, .44 American and .45 Colt. As we entered the smokeless powder era, pressures for the big bores jumped and the use of two-leg primers went away, but in low-powered cartridges like .32 S&W and .38 S&W it hung around, because those cartridges could still work well with it. Vintage ammo collectors and old Winchester patents (circa 1890s–1920s) show anvils in such ammo as simple two-leg pieces, actually just a piece of metal bent into a V or U shape.
Early Remington small pistol primers – before the 1½ and 5½ designations solidified post-World War II – occasionally featured two-legged anvils. Reloading manuals such as early Lyman editions, and collector photos, show Remington anvils as a two-prong design for some cartridges and lots.
The way “two-leg” primers were put together is actually pretty interesting. Before we start, let’s get our terminology straight. As I define it, the “top” of a primer is the smooth, solid portion, the “bottom” is the open end we can look at and see the anvil. Clear? Good.
With three-leg primers the priming compound is applied first, then the anvil is pressed into place over that. That’s not how two-leg primers were constructed. The two-leg primers started out, not as a cup with a closed end and an open end, but rather as a cylinder open on both ends. The anvil was installed first. Think a narrow strip of metal, with the bend point, the pointy part of the V, or the rounded part of the U, being wide enough to serve as an anvil. That was inserted into the cylinder with the open “legs” of the anvil facing down. This left two open areas, one to either side of the anvil, for flame to pour through. THEN the priming compound was applied, over the anvil. Finally, a thin foil or paper cap was crimped into place over top of the primer. This was what the firing pin hit. This gave us essentially a two-piece cup on one end, with two flame holes, one to each side of the anvil, on the other.
So why two legs – or rather a bent piece of metal that gave in effect two legs? Because it was simpler therefore cheaper to manufacture than punching three holes in a normal primer. The bent metal/two legs approach required less metal that three-prong designs, fewer stamping operations, and in low-pressure loads (less than 20,000 psi) like the .32 S&W at 12,000-13,000 psi, and the .38 S&W at 14,000-15,000 psi even with smokeless powder, worked acceptably well. These cartridges didn’t need the extra structural support of a third leg to stabilize the anvil against shifting with the firing pin impact. Said stability ensures a forceful, consistent application of flame to the powder (more about which shortly).
Ammunition manufacturers abandoned the two-leg primers long ago. Because, to answer the question of “Why three legs?” the answer is “stability.” Three legs stabilize the anvil, holding it in position, and resisting the firing pin’s impact, thus ensuring a large, even flow of flame through the anvil’s holes and flash hole, in a way the simple bent piece of metal/two legs design couldn’t. Think the stability of a bicycle versus that of a tricycle. Two-leg anvils could shift under a firing pin impact. This meant, with only two exits for flame, one or both of them could partially close, leading to flame hitting the powder unevenly instead of a large amount of flame hitting the powder evenly.
This became important as cartridges that routinely exceeded 20,000 psi (for instance, standard pressure 9mm at 35,000 psi) became the norm because the powders used in such ammo require more, and more consistently applied, flame to ignite evenly. With black powder, or the powders used in low-pressure loads, this didn’t matter. With the sorts of powders used in high-pressure cartridges, we want the powder getting a forceful, consistently applied gout of flame, otherwise we can get inconsistent ignition, with resultant issues with variable powder burn, huge velocity shifts, and poor accuracy. The three-leg design, resistant to moving under firing pin impact, gives us the stability we need, and ensures three equally sized jets of flame exit the primer simultaneously.
Over time, as the use of low-pressure cartridges waned, it simply wasn’t cost-effective to be producing two types of anvils. Still, the “two-leg anvils” are a fascinating, if obscure, part of ammunition-making history.
