If it's to be an "international" gun, I'd suggest something chambered for a load you can actually find outside the United States. That invalidates .45 ACP, .40 S&W, and .357 Magnum for the most part.
If you're playing Mr. Secret Agent, you probably want handgun ammunition you can get without attracting attention. That pretty much leaves .22 LR.
Sometimes it’s nice not to destroy the world for a change.