Left to right: 10mm, .40S&W
My father purchased the contents of a storage building a few years back and as I was helping him clean out the building and empty the boxes I found a few boxes of 10mm rounds. being someone who collects different rounds I come across I put a few in my collection and put the rest of the rounds in my safe.
The 10mm and the .40 are very similar in most every way but in length of the case (brass).
Left to right: 10mm, .40S&W
The .40 s&w was designed after The FBI tested the 9mm and the .45ACP to replace their .38 spl revolver with a semi auto pistol because it could carry more rounds and would be easier to reload in the field. during the testing the Special Agent in Charge of the FBI Firearms Training Unit John Hall included the 10mm round, using his personal Colt Delta Elite 10mm semi-automatic pistol.
The FBI contracted Smith & Wesson to design a handgun to FBI specifications, based on the existing large-frame S&W 4506 .45 ACP handgun.They found that the could use less powder and still meet the velocity specifications and They also found that by shorten the 10mm case enough it would fit within their medium-frame 9mm handguns.
Smith and Wesson teamed up with Winchester to develop the .40 S&W and when the gun and ammo was released it was an immediate success with both law enforcement and the general public.
Left to right: 10mm, .40 S&W (charts from Wiki)
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[Special thanks to a Misplaced beast]