My dad used to refer to "cooked" linseed oil as a great coating to protect metal. He pointed out he didn't mean "boiled" because that's the way linseed oil was prepared. The "cooked-on" finish was attained by heating the metal to "open it up," brushing or dipping it in linseed oil, giving it time to "set in," then baking it in an oven to "cook" it, leaving a black finish almost impervious to rust.
He'd learned this method in the South (I was born in Arkansas). He showed me a variation to season cast iron pans, a style I used to this day. Heat the pan, rub it with cooking oil, let it set to let the oil penetrate, then bake the pan or cook it on the top of the range until the oil "smokes off." Wipe clean and you're ready to cook!
Like Highlander 73, I have to wonder if this old method was not used by Southern gun builders, to essentially season the metal parts just as you would a cast iron pan. Cast iron pans, properly seasoned, are impervious to rust, a trait that would be valuable in a gun. And the black coating left by the seasoning operation is an attractive finish, easily renewed by a light coat of oil.
My dad used this method on hand tools he used a lot outside, including his saws. Rust was seldom a problem on these treated tools and if some did appear, you just sanded it down and reseasoned it and it was good for another couple of years.
Reading the other posts, I did remember dad saying that hog lard was the old standard, though that was not easily available in Connecticut, where we lived at the time. Dad was a painter, so linseed oil was cheap and available.
But I have used fatty bacon and gotten excellent results! In fact, I often get asked to re-season cast iron pans since this method works so well, even on the cheap imported stuff.
David L