Or you can go with a Talon PCP :-). VERY easy to switch barrels, and the cost isn't that high. Air Force barrels are all Lothar Walther, come in 12, 18, and 24 inch, and .177 or .22. Others, like Mac 1, make them in other calibers, .20 and .25 seem to be pretty popular. Apparently a Talon with a 24" .25 barrel is pretty incredible.
Regarding the switch barrel Mendoza that Stephanie mentioned, I can't remember where I saw it, but I have seen a review on it, and it wasn't good. Unfortunately, I don't remember any of the details, other than the reviewers bottom line was don't waste your money.
Hey, one of the best things about multiple calibers is an excuse to buy more airguns :-)! Good luck.
Dave