Back when an SKS was a $200 proposition, it was the answer. If you really want loaner guns on hand, I'd get LE trade-ins that use the same mag as your primary platform (G19, G23 are pretty cheap), and some of the cheap "Sportical" ARs that run around 450-550.
These would also trade a lot better than some of the other options as they are recognizable brands and platforms. Makes the ammo piece easier as well. Entry ARs are basically as cheap as you can get any decent bolt gun these days. I'd stay away from .22LR, because you are really not going to have much effect on anything past 100m.