Overall Case Length is only a Maximum Length Heavier bullets (with same nose profile) are longer and will be loaded deeper into the case, hence less margin of error. I always suggest students save a tray from a box of ammo that allows cartridges to sit loosely in the tray. Put em in nose down and visually check to see if any cartridge looks "short". Parent case 10mm Auto Case type Rimless, Straight Bullet diameter .400 in (10.2 mm) Neck diameter .423 in (10.7 mm) Shoulder diameter .423 in (10.7 mm) Base diameter .424 in (10.8 mm) Rim diameter .424 in (10.8 mm) Rim thickness .055 in (1.4 mm) Case length .850 in (21.6 mm) Overall length 1.135 in (28.8 mm) Case capacity 19.3 gr H2O (1.255 cm³) Rifling twist 1 in 16 in. (406 mm) Primer type Small Pistol Maximum pressure 35,000 psi (240 MPa) Wanna kill these ads? We can help!