The 10mm Auto

The 10mm Auto is an interesting cartridge that has failed to achieve the popularity it deserves. With a case volume about the same as the classic .45 Auto, the 10mm has a maximum working pressure nearly double the .45's and therefore really can do anything that the .45 can and more.

In its full-power version (which is rarely used), it is a potent submachine-gun round. Most of the pistol ammunition loaded in this country is loaded down from the maximum allowable pressures and velocities and is, for all practical purposes, identical in performance to the .45 Auto.

Soon after the FBI adopted the "loaded-down" round in 1989, someone figured out that if the cartridge was shortened by about .14 inch, it could still produce the same ballistics as the 10mm and fit into a smaller frame. Enter the .40 S&W. The .40 S&W has taken over the law enforcement market, leaving the 10mm out in the cold.

Guns & Ammo first printed loads for the 10mm in June 1990. Loading for it is pretty straightforward. Like most straight-sided cases for use in semiauto pistols, the 10mm requires a three-die set. In loading for conventional rifle calibers, the decapping rod also sizes the case neck to fit the bullet so only two dies are needed. With a straight-walled case like the 10mm, the first die resizes the case body slightly more than necessary to fit the bullet and provides the decapping function. It wouldn't be practical to use the decapping rod to resize the front of the straight case, so a second die is used. The second die opens up the case mouth to the right diameter for the bullet and also puts a little bell mouth on the case. This die should be set carefully so that the bell mouth is just enough to allow the bullet to be easily started into the case but no more. The third die in a three-die set is the bullet seater, and it also crimps the case mouth. This crimping shouldn't be overdone either, because most semiauto cartridges headspace off the case mouth and too much crimping can make the headspace dimension somewhat uncertain. Our dies for this month were the same RCBS set we used for the original loads in 1990.

The 10mm, unlike the .40 S&W, uses large pistol primers. Loaded ammo is available from at least six different manufacturers, and new cases are available from Starline, so case availability is no problem. As you can see from the loading table, the powder selection is broad. Almost any medium-speed pistol powder can be made to work.

Our maximum loads produce velocities and pressures somewhat in excess of the velocities and pressures found with most factory ammunition, but they are still within SAAMI specs. For normal pistol work, loads somewhere in between the starting and maximum will probably be the most satisfactory.