10mm Auto

The 10mm Auto was drawn up in 1983 by FFV Norma AB of Åmotfors for a single pistol, the Bren Ten, and it was made deliberately stronger than the concept Jeff Cooper had proposed. The result is a rimless straight-walled case that carries revolver-magnum energy in a length a self-loading pistol can cycle — and a round whose commercial history is mostly the story of people finding that energy inconvenient.

The case is 25.2 mm long and 10.8 mm across the head, both figures much closer to a rifle-derived case than to the pistol rounds it competes with. There is no shoulder and almost no taper: the neck measures 10.74 mm against a base of 10.80 mm, so the case headspaces on its own mouth and the extractor has nothing to grip but the groove. That geometry is what makes the round awkward to load down. The FBI adopted it in 1989 and immediately specified a reduced charge, and Smith & Wesson observed that the same performance fitted in a case shortened to 22 mm — which is the .40 S&W, introduced in 1990 in pistols sized for 9mm. The parent round went on being loaded at full pressure, and the two now share a bullet diameter and a bolt face while differing by 3.6 mm of case.

Its second life is as a hunting and outdoor cartridge rather than a service one. It is the largest common chambering in a full-size polymer service pistol, which is a different proposition from a magnum revolver of comparable energy, and it is one of the few pistol rounds regularly loaded with 200-grain bullets at over 1,000 feet per second.

Gaps on this page: the C.I.P. and SAAMI figures for this round genuinely differ — C.I.P. gives a bullet diameter of 10.17 mm and a ceiling of 2300 bar, SAAMI 0.4005 in and 37,500 psi — and the C.I.P. sheet is the one transcribed throughout, with SAAMI’s pressure noted beside it rather than stored as a second value. The two loads recorded here are both Federal’s, because Federal is the maker that publishes the test barrel length its velocities were measured in; the velocities quoted in the encyclopedia infobox carry no barrel length that can be attributed to any particular row, so none of them is used.