While I have not personally taken the measurements, everyone I have read or spoken with, including reps at gun stores, tell me the 10mm Auto and .40 S&W are exactly the same in bullet dimensions and casing diameters...it is only a shorter brass case...and most 10mm can fire .40 if the gun can cycle the shorter cartridge (which can cause cycling issues with the magazines, ejectors/extractors and feed ramp variables in design). So I tried it, and mine does just fine, even with the crappy alum casing...Since there are others here who have the 10mm Ronin, I thought some might find the experience useful. .40 is currently about $.15 to $.20 cheaper/round on average than the cheapest 10mm ammo.