Most people will suggest a .22 caliber rifle, such as the Ruger 10/22 and this is a great choice. A .22 caliber rifle can take small game as well as larger game such as deer with proper shot placement.
Another advantage for having a .22 Long rifle is the relatively low-report especially when using CB caps and the ability to be effectively silenced with a commercial or homemade sound suppressor aka “silencer”. Just remembered that such a device is illegal without proper government approval and will land you behind bars if you’re caught, and is suggested here for a worst case scenario only or after you have went through all of the legal hoops.
The downside of the .22 Long rifle round is limited range, penetration and stopping power all of which limit the rounds effectiveness when used for self-defense. I suggest a backup handgun chambered for a cartridge suitable for self-defense. I would not go below a 9mm or 38 special and then us good expanding ammo.
Even with a 9mm and 38 special, you should seriously consider using only the +P rounds such as the 115 or 124 grain JHP +P in the 9 mm or 158-grain lead semi-wadcutter hollow point .38 Special +P for defensive purposes .
Your location would also determine weapons choice. For example those bugging-out in grizzly country should definitely consider something more powerful than the aforementioned 9 mm or 38 special.
My first choice for protection against such large game would be a center-fire rifle chambered for 308 or larger. My second choice would be a magnum revolver with a 5.5″ to 7.5″ barrel chambered for .44 magnum or larger.
It is wise to avoid any armed confrontation if possible. Trust me you are not a coward if you avoid the possibility of being shot or having to shoot someone else. You are not expendable – neither are the lives of your family or those in your bug out group. Those with the macho kill ‘em all attitudes will not last very long after the poop hit’s the fan.
With that being said, a semi-auto military style rifle should be considered especially if you are trying to get from an urban area to the country, where facing organized gangs or other threats attempting to block your exit could be a possibility.
An AR-15 with collapsible buttstock or folding stocked AK-47 (for compactness and concealability) could help get you out of a dangerous situation if you’re forced into one, while taking up little space and not adding significant weight to your overall survival gear.