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This is a chart of Magnum handgun cartridges. I may references this image directly in later posts, as it's good for showing scale.
The .44AMP was the biggest hindrance of the Auto Mag Pistol by far, not just from the standpoint of introducing a new cartridge on the market, one which duplicates the performance of an existing one, but more that it was just troublesome.
.308 Winchester cases were used as a parent case, they fit perfectly, just cut off the top to the right length and you can use it. Problem is, this was just not done with a lot of precision in a lot of cases (pardon the pun), and if you take a caliber to each cartridge from an old box of commercial .44AMP and measure them, you'll find that a lot of them will vary by a fair bit, which isn't good for reliability.