The first “10mm” was the .40 G&A and was actually much more like a .40 S&W. It was a converted Browning Hi Power (so short case, not the longer 10mm case). Barrel was made by Bar-Sto and RCBS made the dies. Cases were made from cut down .224 Weatherby Magnum cases and it even headspaced off of the belt. The original ballistics was nearly identical to what we have now, 180 grain bullet at 1,000fps (today’s load is typically 950fps).
Then near the end of the ‘70’s the cartridge broke off into two camps. One went on to develop the 10mm, the other created the “centimeter” cartridge by holding to the original concept of a modern .38/40; this would eventually be the .40 S&W cartridge.
So in reality, the 10mm cartridge was created from the .40 S&W, not the other way around.
