Some have said Toilet paper would be thier choice. But vegetation and water do as well as toilet paper, albeit with more hassle. Some have said Fire., but if you have a gun and ammo you can make fire easily (I will explain at the end of my post). .308hunter would take his GPS, but what happens when you run out of batteries? (maybe a compass woud be better? But only, I guess if you had a map or some idea of the terrain.
I also thought about a stone to sharpen the knife. Or a tarp. Also I was considering fishhooks and twine.
But, when it comes down to it, cordage is the toughest thing to do without when staying out of doors for a long time. And it is hard to make from natural materials on site.
SO I WOULD HAVE TO SAY ROPE OR CORD! Something small and thin but still with just enough tensile strength to hold me (175 lbs) in case i need to trust my weight to it.
As for the fire from a gun, I have done this several times. This is from Horace Kephardt's "Camping and Woodcraft" first published in 1903.
You take a cartridge (shotgun shells are easiest but you do this with centerfire), and "worry the bullet out with your knife by prying out of it's seat. It is toughbut doable. Pour most of the powder on your tinder, leaving a few grains in the cartridge. Stuff some cotton from your shirt in the cartridge and reload itinto your chamber. Fire into the air. The cotton patch will only go a little way. When you pick it up, it will be smoldering or even burning.. With this you can get a carefully laid fire going.
okcmco
PS. I have to say that a lb. of Columbian roast coffee came in a close second.!!