Maybe not, but a lot of 'em did and nobody really cared. My ex father-in-law brought back his 1911 (we shot the 1945 issued ammo that was in it in 1975. It all worked perfectly, as did the mags that had been loaded since then.).
Another friend of mine showed me the M1 Carbine that he brought back with him when they pulled him off a South Pacific island and sent him to the Naval Academy.
(RIP, Pinky and Dick and Semper Fi)
It just wasn't that big of a deal. Vietnam changed that, because the issue guns (M14, M16) were both select fire. However, I have no doubt that some enterprising GI's managed to do it anyway.