The present invention is a quick connect receiver adapter assembly for an automatic or semi-automatic rifle, specifically the AR-15 assault rifle. The adapter system is used to quickly exchange barrels on an AR-15 rifle platform, and allows the user to convert the AR-15 from a single caliber weapon to a multi-caliber weapon. The system has a receiver attached portion comprising a three piece locking ring assembly, and separate barrel adapter assembly which can be attached to any caliber barrel suitable for use with the AR-15 assault rifle.