Koa is used in The Islands for ukuleles, and there is no finer wood for a uke. It sometimes gets used for other instruments. I've used a lot of it in furniture, mostly as an "accent" wood or veneered panels on Greene & Greene style furniture. I rarely use it as solid wood. Too dear. Decent stuff is very hard to come by, and it is now illegal to cut a live standing koa tree, so what is cut now is either downed salvage wood or standing dead trees from what I'm told. The price keeps climbing higher while the quality plummets. I sometimes run across a stick or two in someone's shop that has been lurking for a long time. Otherwise I'm discouraging clients from using it. There are other woods in the acacia family that are reasonable substitutes, but not quite the same. If you've got any koa, don't let it go cheap!
Works about like black walnut. More open grained. I'd say it's unsuitable for a stock, unless you are trying to make a bench copy of a gun produced in Hawaii around 1750. And if you've got one of those, show us photos, please!