SHMEL, I have a P89 Ruger that is 9mm, I also have a 30 Luger complete slide (with barrel and return spring), for me it is a matter of switching slides. Adap-ter kits are available for the 1911 to shoot 22 ammo, Mausers have an adap-ter kit that allows the shooter to practice? with 22 ammo.
rifles and pistols are available with additional barrels, most are single shot from Thompson. I have receivers, I have barrels, I have no interest in building a convertible, back to single shot, and the imagination getting away from us, the receiver can have two bolts but feeding everything? would be a problem. Then there is removing and installing barrels, conventional thinking requires two wrenches, there is nothing written that says that is the way it has to be done, there are striker wrenches, for tight barrels use a bigger hammer, when using torque hammers the handles should be the same length, meaning, and that would require me to spend the rest of my life to explain,
Sleeves/Inserts, inserts are available, in the big inning an insert was used to modify the 30/06 chamber to 308 Winchester/7.62 NATO, most thought the insert was an either and or, as install the insert and shoot 308W, remove the insert and shoot 30/06 with little and or no thought given to the difference in diameter of the 30/06 and 308 W chamber, the 308 W case had to be sized down at least .011” at the shoulder to chamber. Back to “little and or no thought given” many reloaders and smiths have chambered 308 Winchester/7.62 NATO chambers to 30/06 chambers. After firing they discovered the 30/06 reamer did not remove all of the 308 Winchester chamber, the part of the chamber that did not get removed was the shoulder, a case when fired would have a ring around the case where the old chamber still resided.
One receiver with two bolts and multiple barrels is doable, making sleeves would be too expensive, inserts should be kept in the same family (7MM, 30 CAL, etc.,), I am the fan of the running start, I want my bullets to have a jump start, I have no interest in an insert that increases the distance between the bullet and rifling, but, it is possible to form cases with long necks, something like throating.