Short Frame. The good folks at Glock decreased the dimensions of the backstrap slightly making the G21SF(45ACP)an easier gun for those with average hands to shoot comfortably. I think they've done this with the G30 and G36 the other 45ACP guns in their inventory. The regular G21 was a "fat" pistol in the backstrap laterally(from web of hand to trigger finger)so this was a welcome move.
Actually a lot of the controversy between "slim" and "short" comes from the original description of the G36 and the new descriptions of the G21, 20, 30, and 29 models. When the 36 was originally marketed...it was advertised as a "Slim Frame" due to its new single stack design. The newly designed "less long from backstrap to trigger" models are "Short Frames"
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