Welcome LCranston.
Talk about resurrecting an old thread.

Yes the over under in two calibers was a decent pattern for a survival rifle. It gave you the choice of two different loads to match the game and the ranges you were likely to shoot it at.

It was a good enough pattern that the Royal Canadian Air Force used to use it as the Survival rifle for transport planes operating north of 60.

I am not familiar with .243 for range or knockdown myself. It gets good reviews though. It might not be quite all you might want for moose or bear, but it seems to outperform the 5.56 NATO/.223 Remington for long range varmint or dear hunting.

I hope you can find the ammunition easily.

The 20 gauge shotgun ammo should be easy to find and should be good on anything up to the size of a fox, or for birds from dove through partridge to ducks, out to about 50 yards (depending on shot size)
So you have both long and short range covered quite nicely.

Good luck with your new rifle.
Remember to practice with it before you need it.