My issue gun from the Air Force was a S&W M-15, Combat Magnum, and the issued rounds were 130 gr FMJ, .38 Specials.
Just a minor point, but the M15 is a Combat Masterpiece rather than a Combat Magnum as the M15 was only ever chambered for .38 Special. The most similar gun that could be called a Combat Magnum would be the M19 which was based upon the M15.
This does bring up another interesting point. If one did take a personally owned .357 Magnum revolver to war, it could possibly be used with military-issue ammunition as .38 Special ammo had been in the supply chain since at least WWII if not earlier.
Another interesting tidbit is that the 130gr FMJ .38 Special loading used by the USAF was a very light loading. It was intentionally loaded very mildly out of deference to the M13 "Aircrewman" that was purchased by the military. The Aircrewman is not to be confused with the later M13 in .357 Magnum as the Aircrewman was, for all intents and purposes, a M12 Airweight fitted with an Aluminum cylinder. I've read that S&W currently considers the Aircrewman unsafe to fire and that, if one is sent to S&W for any sort of repair, it will return with a new steel cylinder fitted. Because of this and its low production numbers, the Aircrewman is a rather valuable collector's item worth a significant chunk of change.