Got the mag in front of me right now. Gun World claims that it is a 10mm +P. It is a wild catridge jointly developed by Texas Ammunition Co. Ballinger, Tex
www.texas-ammo.com , and Ace Custom .45s Kerrville, Tex
www.acecustom45s.com. GW traces the early development of the 10mm by Whit Collin, then call 40G&A using cut down .30 Rem brass and 180gr 38-40 bullets to Tom Dornus' and Mike Dixon's ill-fated Bren Ten.
The concept was to develop a cartridge with characterists of 9mm Para and the stopping power of the venerable 45ACP, kinda of like the 40S&W of today.
Further GW claims that in today's downloaded 10mm, they are not even close to the original Norma 10mm loading. They claim that the 10mm Express exceeds the original Norma 10mm loadings. The 10mm Exp most powerful loading is a Hornady 200gr FMJ 1,250 fps/694 ft-lbs.
Apparently Ace Custom built a 1911-based pistol using a lot of custom parts and a propritary recoil spring system.
The accuracy was 2.375" at 75' using a Sierra 165g at 1450 fps/630 ft-lbs.