The CEF had Colts when they first arrived in France - they were found to be prone to jamming and were replaced with the more reliable Vickers around 1916. I agree that the Vickers would be more useful: due the nature of trench warfare, the CMMGB tended to dismount their MGs for action, and about the only time the Autocars saw actual combat was during the German spring offensive of 1918, when they mounted Vickers. The rest of the time, the Autocars were mostly used to transport the MG brigade around.