Gun enthusiasts enjoy being able to shoot different rounds. Typically, this requires purchasing a firearm for each desired round type if firearms are only capable of firing a fixed round. However, having separate firearms is very expensive and becomes cumbersome when shooting different rounds. Therefore, the gun industry has moved towards modularization of firearms. For example, U.S. patent application Ser. No. 2012/0167433 discloses a modular automatic or semi-automatic rifle that includes a lower receiver configured to mate with a 7.62 mm NATO compatible upper receiver and to mate with a 5.56 NATO type upper receiver via either an interface adaptor inserted via a top of the lower receiver in a magazine well or alternatively, via an integral adapter mounted or assembled to a frame of an upper receiver. These adaptors are required in order to convert between the two different rounds. While this modularization of firearms allows shooting different rounds, it requires additional external parts that add complexity to the reconfiguration, increases the risk of loss parts, and increases the amount of equipment required.
The present invention is directed at a convertible lower receiver that requires no additional external parts when configuring from one round to another.