A system and related methods for interchanging barrels on a gun and packaging such components is described herein. Preferably, the system includes a barrel having a first end, a second end, and a shaft between the first end and the second end. In one embodiment, the barrel includes a clamping screw notch and a barrel indexing surface formed on the shaft. The system further includes a breech block having a barrel housing formed therein having a first end, a second end, and a hollow core therebetween for receiving the barrel, and an indexing pin positioned to engage the barrel indexing surface upon receipt of the barrel through the barrel housing. The first end of the barrel housing is adapted to engage the receiver of the gun. The second end of the barrel housing includes a clamp and a clamping screw orifice formed therein for receiving a clamping screw. Preferably a portion of the clamping screw extends partially into the barrel housing to engage the clamping screw notch upon receipt of the barrel through the barrel housing, although alternatives such as a set screw are also disclosed. Tightening of (1) the clamping screw within the clamping screw orifice or (2) the set screw removably joins the barrel to the barrel housing.