I used a vinegar and dissolved steel stain on a couple of maple stocked rifles I built and people were always commenting on how beautiful those walnut stocks were. With the vinegar stain, they do look like walnut, but not a dark color, more like a light brown walnut.
The thing with the vinegar stain is, it needs to brew for at least a month or two. The longer it sets the better. I start out dissolving steel wool in a jar full of vinegar. Once the steel wool has dissolved, I throw in a handful of rusty nails or such. I keep the lid off so that some of the liquid evaporates, then add more vinegar so that the stain gets more concentrated as time goes by. It's good to test on a piece of wood that came from the same stock you want to stain, or a spot on the stock that doesn't show, like the barrel channel or lock mortice. Don't test on the wood behind the buttplate because the end grain will show darker than the side grain. No heating involved.
The aqua fortis will make it as dark as you want, just keep adding applications and then don't nutralize. But I think it tends to have more black color than most walnut. Bill