>Any weapon which shoots, is designed to shoot, or can be readily restored to shoot, automatically more than one shot without manual reloading, by a single function of the trigger

Technically, that makes an SAA (or single action revolver that I know of) a machine gun. You can empty all 6 chambers with one pull of the trigger. Granted, you have to manually manipulate the hammer, but the ATF definition doesn't say anything about the hammer, just the trigger.