My advice would be to forget it. You can buy an auto sear legally, as Blades67 says, but many are of poor quality and do not allow selective fire, that is, once installed they are full auto only.
If you can find a functioning full auto CAR, and can afford the ticket, that would be the way to go. Converting or attempting to convert in any other way is illegal.
In addition, many (most) ranges will not allow full auto fire for both PR and practical reasons.