I know this goes a bit off what was originally asked for and I'm being a bit of a prude on this...
Inexpensive switch barrel combo, supporting multiple calibers, case head sizes and case lengths?
Light and easy to carry when stored? Requires no need to re-sight-in after barrel changes?
Try:
http://www.hr1871.com/firearms/index.php?cat=4&subcat=36
You can get a complete Handi-Rifle for under $200; I walked out the door with a synthetic-stocked model for $170 at my local dealer (nice sale). Extra rifle barrels go for $82 for blued on the web site. You can even add a shotgun barrel to the rifle reciever for $59.
The base rifle, an extra rifle barrel, and an extra shotgun barrel won't set you back more than $350. With two scopes, the entire package should fit into a briefcase-sized guncase no longer than 24" (with 22" barrels). That's portability. And no funky tools to change out barrels; all that is needed is a stubby phillips-head screwdriver to remove the forearm.
I have never read a negative review on the Handi-Rifle. The trigger is very good, and you can't get any simpler in operation. You can't get any more compact, save a bull-pup design. The handling and carry characteristicts are sheer joy. A setup with .223 Rem, 30'06, and a 12 guage barrel would be more than adequate for anything on the Continent.
I know that this forum is about overcoming challenges and creativity, but sometimes there are simpler solutions...