HK fashioned the HK P7M10, in .40 S&W, from the high capacity 9mm version of the P7, the P7M13 (which means 13 rounds). The original P7s, by the way, hold 8 rounds in the magazine. HK cooked up the 13 round version for the U.S. Army pistol trials back in the mid-80s. After the Army rejected it, HK marketed the high capacity P7 commercially to law enforcement and private citizens until the onerous 1994 Crime Bill. Now, HK lists the M13 as law enforcement only. As for the M10, well, some people think that it would have been a better gun in a single-stacked rather than a double-stacked magazine. For me, the double-stacked versions, whether in 9mm or .40, don't fit. You gain much by losing a few rounds.
