I know lots of guys that intentionally hunt from a fouled bore with great success. I also know that reloading on a fouled (or spit/dry patch) bore is also more difficult
Everyone has their own procedure that works for them.
With respect to a spit patch only. Your saliva has salts in it which can leave a flash rust in the bore during a day's hunt. Which is one of the reasons I don't use the spit patch onlyprocedure for hunting, only for target fun.
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loading on a fowled bore difficult? ...
You need my White Rifle. I swear the more dirty the bore gets the sweeter it shoots... Also I fowl my Knight Wolverine, then swab it before I hunt, as I get more consistent accuracy off the fowled bore. Why my Disc is not like that I have no idea. How many I wonder actually fowl their bore before going hunting? Might be an interesting poll.
As for the spit in a person's body, while I am sure it contains body salts.. I wonder if the concentration would be enough to pose a danger of rusting in a days time? That might make for an interesting experiment for someone...