Getting "lost/trapped" you don't have much choice. So lets just say is I were to have a choice I would probably hope for a .45 as I could carry more ball and powder - or make what I had stretch further.
For small game I'd hope for a couple six foot lengths of wire to fashion a couple snares and check them in the mornings to save even more powder.
When I go hunting (big game or small game) I carry a pretty full kit, and the same gear regardless. Trying to get the feel of what it was like. One of my favorite "hunts" is to head out and pop a squirrel and then prepare it where I took it. Roasted on a green stick over a flint & steel struck fire with a cup of hot tea. Good stuff.
Here are Carp and I as we deer hunted last Fall. I've got the fixins for 40 shots, as I usually do. This is about the same kit as I would have for a fun shoot or a squirrel hunt. The rifle is a .54 The haversack has enough fixins to set up light camp and hold out for a week. I'd be missing blankets for sure, but I'd be alive and alert. I'm out of parched corn but have a half pound of rice and some jerked venison, a billy cup (corn boiler) and cord, tinderbox, compass (a flashlight and quart of water - I'm not suicidal), folding knife and a dry pair of heavy wool socks (aka "pot mitts, mittens, berry buckets). Also the waxed cows-knee for my flintlock.
The hunting pouch has only what I need to shoot and clean the rifle.
When I hunt during regular season I often carry a .50 T/C New Englander and a belt pouch with seven shots in a ball block and pre-measured powder in tubes. I don't consider that "proper" for a primative season but it is a fine, sporting firearm (I can't use a C.F. rifle and a muzzleloader with round ball is much more accurate than a slug shotgn). That's as much a practical concession as a sporting one.
But, for extended survival, give me a .22 WMR in a bolt action and a couple hundred rounds. I'm gonna have too much to do to worry about hunting and using my hands for gathering to carry along a heavy gun, and I can pop what opportunities come along with that. ;-)