FBKampfer
Banned
My guess is that the British and French guns were available in bulk sooner, so could equip the new divisions more quickly than ramping up US production. Secondly, if you're going down that path of buying foreign guns then you might as well build their designs on your own production lines, to match what you already have in service. I think shipping concerns and ballistc performance would come after that.
The question is then raised, where would the US fight if they joined the CP? Canada is a certainty, but it would be tough for the US to directly aid Germany in Europe.
Once the Russians are out of the fight (and who are we kidding, keeping the Russians a going concern is a herculean task in itself), it's possible US troops could be shipped through Russia.