The 6 x 45 mm is the only cartridge I know based on the .223 case that is practical for use in the AR. It reatains the body taper of the .223 Rem. and just necked up to accept the 6mm/.243 bullet. It can be loaded to magazine length, but gives up a lot of case capacity.
The TCU family of cases (6mm, 6.5mm, and 7mm TCU) are not practical for use in the AR. They have a blown out body with very little taper and will not fit through an AR-15 magazine. They are loaded to considerably longer OAL than the .223 Rem. magazine length cartridges.
The "Whisper" cases are based on the .221 Fireball case and can be loaded to magazine length. And will readily feed through an AR mag.
Regards! DaMan
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