G'Day Fella's,
Johnler, a switch barrel gun is just that!
It is a Bolt Action rifle that has been set up with a series of barrels that can be screwed out and another screwed back in as you feel the need.
I have a Remington 700 bolt action that was trued to benchrest standards, and I have a couple of barrels that I swap depending on what I'm shooting at.
Mate, I'm a bit rusty on AR's but if I remember correctly, the barrel retaining nut on these rifle holds and clamps the barrel extension in place in the upper receiver. So as long as you have cartridges that use the same bolt head (.223, .300 Whisper and others), you should be able to swap and change at will! Just remember to remove the Gas Tube!!!
I'm sure someone will shortly respond and confirm this, one way or another but I hope that helps some amount!
Doh!
Homer