The 6mm TCU was developed by Wes Ugalde of Fallon, Nevada for Thompson/Center as one of the chamberings for the Contender single shot pistol. It is the 223 Remington necked up to to 243 caliber and is practically identical to the older (1965) 6mm 223 Remington or the 6x45mm. The 6mm TCU is intended primarily as a metallic silhouette pistol cartridge, whereas the 6mm 223 Remington was developed as a rifle cartridge for benchrest and varmint shooting. The 6mm TCU is adequate for small to medium varmints, but is on the light side for deer. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.