If you do decide to use real nitric acid, formula HNO3, pay some attention to where you store it
It wold be a good idea to wear a face shield when handling it. A tiny drop will ruin your binocular vision.
Even in a closed bottle it will give off fumes that thoroughly rust anything you don't want rusted. Like guns, good tools, your old clock &c.
Yeah, I learnt this the costly way, years ago.
Nevertheless, if you wipe your stock with it, then heat it, it will oxidize (burn) to wood to make an interesting color. If you do this with the brass inlays still in the stock, after some months you will notice the brass looks like copper around the edges. This is because traces of that acid slowly remove zinc from the brass. I learned this one by looking at the work of a very fine maker in the East, maybe at the Baltimore show, in the 1970's. Disremember exactly who and when.
Along with chemistry class.