If the wood grain has no serious defects to hide, Spar Varnish over bare wood would be my choice. I would skip the stain as the Spar Varnish will darken it a bit anyway, and it will continue to darken as it ages over time.
The reason that I hate plain ole paint is that it doesn't last forever and I hate scraping/painting over and over again. That is not to say that Spar Varnish lasts forever, but a bit of mild sanding to remove deteriorated bits of it followed by another coat will bring it to "new" all over again.
Maybe you are lucker than I and have an inside place to store your gun. I lack a garage and the living room is off limits to cannons, so they sit out in the weather all the time. The best I have been able to do is put a plastic folding card table over my naval gun and an upside down plastic storage container sits over my mortar.
Rick