The roots of the 10mm go all the way back to the last quarter of the 19th Century when Winchester necked down the .44 WCF (Winchester Centerfire] to .40 and called it the .38 WCF. The .38 WCF is more commonly known now as the .38-40 which adds to the confusion as it is not a .38 but actually a .40. The .38-40 was the third most popular chambering found in the Colt Single Action Army. Fast-forward to the 1930's and we find wildcatters using the .38-40 Colt 5AA with new cylinders chambered in such cartridges as Pop Eimer's .400 Eimer and Gordon Boser's .401 Special respectively. Both of these cartridges, as did the .38-40, used bullets measuring 0.400-inch or 10mm in metric.
In the 1970's we have the modern version of the 10mm beginning with a wildcat cartridge known as the .40 G&A. This cartridge used .224 Weatherby brass cut to the proper length and 180-grain .38-40 jacketed bullets which were at the time available from Winchester. Velocities were right at 1,250 feet per second and the cartridge was chambered in a Browning Hi-Power. I gotta believe this cartridge put a lot of strain on the Browning!
In 1984 Jeff Cooper--a staunch advocate of the .45 Government Model 1911--put his stamp of approval on a new semi-automatic pistol and cartridge from Dornaus & Dixon. This double-action semi-automatic could be carried cocked-and-locked, and was basically a CZ-75 9mm with a larger hole in the barrel--a 10mm one. Cooper gave it the catchy name of the Bren Ten. There was a lot of interest in the Bren Ten, however, financial and production problems surfaced. Some of those who got pistols could not get the magazines. By 1987 the Bren Ten was gone and Dornaus & Dixon declared bankruptcy. That could've been the end of the 10mm.
The 10mm followed a path parallel to the .41 Magnum sixgun cartridge. Both of these were originally conceived as self-defense/LE cartridges and both proved to be too powerful for such use by mere mortals. However, both cartridges are excellent outdoor cartridges. That is, cartridges used for hunting or for just a very comfortable feeling when carried on the hip as you go off the beaten path.
The Bren Ten wasn't the end of the 10mm as Colt came to the rescue by chambering their 1911 Government Model in 10mm, known as the Delta Elite. The original loading for the 10mm, was a 200-grain bullet at 1,200 fps. At the time I believed this was too powerful for continued use in the 1911 and...