"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.