What caliber would it be?

If I could only have one long gun hands down it would be my 22 mag. I trust it, and am more confident with it than any other gun 77/22 all weather, 3x9 VX2, with a trigger job. I think its capable and reasonably practical for most applications, with good shot placement 40 grain jacketed hollow points its deadly on anything out to 60 yards probably further. And at close range I'm always impressed with how devastating it is for such a small shell. Last but not least Ammo is still pretty cheap and you can carry a lot of shells without taking up space and weight.

If you needed one gun in a survival situation regardless of the circumstances this would be my pick.