The thing that makes gun carving trickier than furniture is that, generally, our American Long rifle carving is done in low relief, and furniture is done in high relief. The trick with low relief (max of 1/16" but most of it is much lower than that) is to make it LOOK like high relief in executing it. Under cutting and trenching around the the features so stain and finish can get under there is one way to make it appear higher. When you do high relief stuff you don't have to do that stuff as much because the features themselves are generally larger and don't need it as much.
From the examples I've seen over the years on this forum is that a lot of guys fall down on leveling their background planes around their features. You can often see the "digs" around the features. Scrapers and very small profile sanders can be very good tools for helping to get rid of it. I did too my first few attempts. Yeah, it's boring and time consuming. As mentioned previously, strong light from many angles can often help reveal it. Patience is a must have.
Another good reference book is Shipper's book "Engraving Historical Firearms." Lots of well done carving in that book to take references from (as well as the engraving).