A 20" diameter tree isn't much to go with. The light colored heart wood will never get dark enough to be any good for gunstocks. The closer you can get to the ground the better wood will be near the roots, and near where braches were. The rule of thumb in drying it, a year per inch thickness and use any old paint, the thicker the better, to coat the ends of the boards, so that the moisture does not leave the wood too fast. Sticker the wood with walnut strips about 1"x1" x width of stack and cover it. If you want it to dry faster, and more evenly there are sites to look at in making your own inexpensive solor kiln. Whatever process you use the final moisture content should be between 6-8%. If the wood is going to be dried outdoors, when it gets close to time, bring a piece that you want and let it sit indoors to aclimate to the surrounding air temperature to stabilize that 6-8% moisture content. Plus you will need a moisture meter. I use a Lignomat, Mini-Ligno 6-20% ($100.00) that has pins, and gives you a chart so you can flip the switch for hard and soft wood readings.
Good luck and keep us posted on your results.
