10mm Magnum

A lineup of various handgun cartridges, including the 10mm Auto (center), and the 10mm Magnum shown just to its right.

The 10mm Magnum looks to be a fairly obscure centerfire pistol cartridge, with origins appearing to go back to Arcadia Machine & Tool (AMT), in a version of their Auto Mag series of pistols - (the AutoMag IV? It's unclear if the cartridge has ever been referred to as the 10mm AutoMag, so that needs confirmation.

The cartridge is (mostly) a lengthened 10mm Auto, made to achieve significantly higher velocities, and to be suitable for hunting, competition shooting for silhouettes, and so on.

It should be noted that a longer case length is not the only change to the case. It also has a thicker internal portion of the rim and flash hole, as is shown in the image second down on the right of this article.

A case comparison between the 10mm Magnum (left), and it's smaller brother, the 10mm Auto (right). As you can see, the length of the case is not the only difference.

Experimentation by Colt

Colt seems to have briefly experimented with this round in their Colt MARS "Mini Assault Rifle System" PDW program in the late 90's, by necking down the cartridge to accept a .355 caliber projectile, creating their 9x30mm MARS cartridge. This round was the counterpart to their other experimental PDW-style cartridge, the 5.56x30mm MARS, which was essentially a shortened 5.56 NATO round that fired at reduced velocities to be suitable for their new design's smaller form factor and lighter-weight.

Ammunition Availability

It doesn't appear there is any clear or simple way to create casings for the 10mm Magnum, though ammunition does seem somewhat available from Reed's Ammunition