The 6mm-223 Remington, also known as the 6x45mm, came into being in late 1965, shortly after Remington introduced the 223 Remington as a sporting round. After its brief flurry as a benchrest and match cartridge, the 6x45mm has now been relegated primarily a varmint cartridge, used by those who want more power than the 223 with the added advantage of being able to use cheap military brass for forming cases. In power, the 6x45mm is between the old 25-35 and the 250 Savage, which would make it rather marginal as a deer cartridge except under ideal conditions. It is, however, as close to ideal as a varmint and small game cartridge out to 300 yards. This cartridge has become very popular in the Thompson/Center Contender and Remington XP-100 handguns. Bob Milek, the first editor of Guns & Ammo and Peterson's Hunting magazines shot a custom XP-100 in 6x45mm for a number of years. Rifles chambered for cartridges in this group are pleasant to shoot, have relatively low report and are noted for long barrel life. 

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