Dan You are correct!!! It is a cutdown and necked ,223 round the standard 22TCM will not work in a 9mm magazine and that is why the RIA conversions use the 22TCM9R cartridge which has a shorter bullet to allow it to fit the 9mm mags. You can use the 22TCM9r in standard 22TCM guns but you can not use the 22TCM in a 22TCM9R gun "The .22 TCM or 22TCM (.22 Tuason Craig Micromagnum) is a proprietary bottle-necked cartridge created from a 5.56 NATO cartridge developed by custom gunsmith Fred Craig and Rock Island Armory (RIA) for semi-automatic pistols and the Rock Island M22 TCM bolt action rifle. Before the cartridge was commercialized, it was called the 22 Micro-Mag. Similar conceptually to other bottle-necked pistol cartridges such as the larger-caliber .357 SIG, the .22 TCM trades bullet mass for increased velocity and lowered recoil.[1]
Based on the 5.56×45mm NATO case and shortened so that the shoulder is at approximately the same length as a .38 Super cartridge, the .22 TCM is somewhat longer than the ubiquitous 9×19mm Parabellum and designed to be fired from a RIA line of weapons (which also included 9mm barrel swaps) fed by Para-Ordnance-style double-column .38 Super magazines. A sub-variant, the 22 TCM 9R, with a shorter, more deeply-seated bullet, is designed for use in Glock magazines limited to standard-length 9mm cartridges. Standard factory loads are 40-grain jacketed soft hollow point, 39-grain for "9R". "

Dan You are correct!!! It is a cutdown and necked ,223 round the standard 22TCM will not work in a 9mm magazine and that is why the RIA conversions use the 22TCM9R cartridge which has a shorter bullet to allow it to fit the 9mm mags. You can use the 22TCM9r in standard 22TCM guns but you can not use the 22TCM in a 22TCM9R gun "The .22 TCM or 22TCM (.22 Tuason Craig Micromagnum) is a proprietary bottle-necked cartridge created from a 5.56 NATO cartridge developed by custom gunsmith Fred Craig and Rock Island Armory (RIA) for semi-automatic pistols and the Rock Island M22 TCM bolt action rifle. Before the cartridge was commercialized, it was called the 22 Micro-Mag. Similar conceptually to other bottle-necked pistol cartridges such as the larger-caliber .357 SIG, the .22 TCM trades bullet mass for increased velocity and lowered recoil.[1] Based on the 5.56×45mm NATO case and shortened so that the shoulder is at approximately the same length as a .38 Super cartridge, the .22 TCM is somewhat longer than the ubiquitous 9×19mm Parabellum and designed to be fired from a RIA line of weapons (which also included 9mm barrel swaps) fed by Para-Ordnance-style double-column .38 Super magazines. A sub-variant, the 22 TCM 9R, with a shorter, more deeply-seated bullet, is designed for use in Glock magazines limited to standard-length 9mm cartridges. Standard factory loads are 40-grain jacketed soft hollow point, 39-grain for "9R". "

Dan, your correction is correct. The .22 TCM or 22TCM is based on the 5.56×45mm NATO case and shortened so that the shoulder is at approximately the same length as a .38 Super cartridge, the .22 TCM is somewhat longer than the ubiquitous 9×19mm Parabellum and designed to be fired from a RIA line of weapons (which also included 9mm barrel swaps) fed by Para-Ordnance-style double-column .38 Super magazines. A sub-variant, the 22 TCM 9R, with a shorter, more deeply-seated bullet, is designed for use in Glock magazines limited to standard-length 9mm cartridges. Standard factory loads are 40-grain jacketed soft hollow point, 39-grain for "9R". https://en.wikipedia.org/wiki/.22_TCM#/media/File:TCM_FN_Rem.jpg