Some of you guys clearly have never served in the military, even in peacetime. Colt Python? Give me a break. That would have lasted about two days in Vietnam. Maybe a week in Iraq?
When I was in the 1ID, I was authorized to have no handgun, only the M-16, and I'm hard pressed to think of a time I would have preferred to use the M1911 over the M-16. Remember, the handguns used in Korea and Vietnam were all made during WWII, so they were not exactly new items. I do own a 1911 now, a Colt XSE, and of all the handguns I own or have owned, it is the one I shoot best.
Other practical shooters here have suggested the G19. I sold my G19 three years ago and have regretted it ever since. It is easily the best all-purpose (concealed carry, home defense, target shooting) handgun ever made. Small, light, yet still with a 4" barrel, full 3-finger grip, and 15 rd magazine.
However, if I were back in the army, limited to FMJ ammo, I'd lean toward a 45ACP handgun. And as much as I like shooting the 1911, I think I would have to go for hi-cap with something like my XDM-45 4.5" with its 13-round magazines. 14 shots of 230gr lead before a reload. Like the army execs, though, I'd add a thumb safety in addition to the grip safety of the XDM. I like the S&W M&P-45, but that 10-round mag that really only takes 9 in most samples, is a downside, although it is available with a thumb safety.
And I sure would like to have had a dozen 30-round Magpul mags back in 'Nam...along with that XDM45.