Sometimes it's not only fun to play these hypotheticals, but I think they are also important for the purposes of open discussion to gain ideas and advantages should a "situation" ever come to grasp.
However, there are no details to your hypothetical and the situation really does matter as to what tools or weapons one would use in any particular situation.. Is it a nuclear strike? Are there huntable poulations of gameanimals and fish or maybe even access to food storage or grocery stores, first aid supplies, water, etc..? Is the air breathable, or is there an airborne virus that wiped most people out? Did the earth tilt, was there an ice age, or is it more of a civil disorder situation like an earthquake and rioting?
In any earth ending or major unrest situation, a large caliber revolver would not be a suitable survival tool in my opinion but the AK-47 would and could suffice. It is a combat rifle as well as a hunting weapon. I have hunted with an SKS before, so thiscaliber has some added value and is proven in an array of gross environments.
In most cases, I would say an accurate, well-constructed,combat semi-auto handgun, and a rifle capable of firing many rounds per magazine and still able to provide enough punch to take game and bea popularcaliber for easier-to-find ammunition.