The term has been used interchageably for many years. The early M-16 carbines were known as CAR-15's. The old Mil designation XM-177E2 was a bit cumbersome. IMHO CAR stood for "Carbine Armalite Rifle". Remember, the first M-16's were stamped AR-15. Some were stamped with both AR-15 and M-16. Today the "Cool" term is M-4. Basically the same thing.