.22 "Takedown" Rifles
My second choice for 'only one gun' would be a .22 rifle. Good utility, reliability, compact, and you can pack a lot of ammunition without adding a lot of weight. Examples are the Ruger 10/22 Takedown, the Marlin Model 70PSS, the Henry AR-7, the Savage Model 64, or IF you can find one, the now discontinued Browning SA-22. Any of these will pack easily in limited space.
Demonstration of the Ruger 10/22 Takedown rifle