Not all Marines were good shots and that is why the 3 levels , Expert , Sharpshooter and Marksman distinguished the difference in your ability with a rifle.
I think that you would probably agree that the amount of training received, and the minimum requirements to qualify, would insure that: relative to the average civilian’s shooting abilities, even the lowest Marine Corps shooting level would be considered a very good shot.
Charlie you're painting with a very broad brush..... Simply because a man became very good with an excellent rifle after intense training and coaching does NOT mean that he would be equally good with a entirely different rifle. ( A rifle that was used by armies that LOST the war)
Simply because a man can drive a modern car with an automatic transmission and power steering, does not mean that he could be equally proficient behind the wheel of a model T Ford.