Without any prior knowledge of the pipe gun, just looking at the magazine selection alone implies to me that you can change out the type of ammunition it can use so I'd assume that if you had a receiver using .45 and its respective mag and swapped the receiver for one using .38 for example then you'd be forced to change out the magazine since that makes sense. Until I saw this art, it never occurred to me that the weapon variation could include caliber/ammo changes, and that's exciting indeed.
And to think, considering how good it seems to already be, imagine how good it can get when a good modder gets his/her paws on this system.