UPDATE: Here is a very interesting option...
"X Caliber is the most versatile Gauge Adapter system in the world. It allows you to shoot 11 different caliber rounds from one shooting platform: a single-shot 12 gauge shotgun. Each X Caliber adapter is made from 7 inches of 4140 chromoly steel. Each adapter is also rifled to increase accuracy.
The calibers of ammunition that the X Caliber can fire include: .22LR / 38 special / .357 / 9mm / 45 acp / 45LC / .410 shotgun / 12 ga. shotgun / .40 cal / .44 mag/ 20ga. shotgun."
(Publisher: I've been watching a Discovery Channel series about volunteers from the city going into the wilds of Alaska to survive weeks on the land while treking from shelter to shelter. What looked to be very useful in their survival situation was a Rifle Shotgun combo that has been sustaining their lives so far. In the last episode they left their larger bore rifle behind due to its weight and opted to carry the one .22/410 combo rifle to keep the final 5 people alive.
I think that I would be willing to leave a lot of other stuff behind before I'd leave my large bore rifle behind in the wilderness of Alaska,... but then I was not there to feel the starvation and the extra weight.
I asked a 'gunnie' friend to do some research for me in case I might like to get one someday and this is what he sent me. I thought you might be interested also.)
Here are a few examples of the various "affordable" combo guns available. I thought there was only one company still making these in Russia, but it turns out Remington is currently making one, at a decent price.
Rossi Centerfire Rifle/Shotgun Matched Pair, Youth Size, 20Ga 22" Mod/.223Rem 22" Blue, Black Synthetic Stock, S20223YBS
The Brno Combo gun is a great companion for predator calling or mixed bag hunts. The 802 Combo gun features 23 ½” barrels, 12 ga, 3” with Improved Modified choke over your choice of .243 Win., .308 Win., or .30-06 Springfield. Features include an automatic safety, extractors, barrel mounted sling attachment, and quarter rib iron sights. The shotgun bbl is fired by the rear trigger, while the front trigger includes a set function for precision shooting with the rifle barrel.
(Publisher: The links below were the result of a search on the site for guns being sold and may not be current. This link is to a search of GunBroker for Rifle/Shotgun Combinations and will produce a current listing)
Here is one that folds away neatly for backpacking.
Here is one that has been common in American hands for decades.
Here is the new Remington offering.
And here is the one I feel would actually be more usefull in the deepest darkest bear-infested woods. It is Russian, and available a wider range of caliber combinations than the others.
Here is a catalog page showing the caliber combos it has been made in.