Acorn caps seem like they could work, you can shape them quite easily too! On second thought, they might be a bit heavy for a cone.. Long thorn through an entire acorn, doesn't seem like it would work out to well... Though for a long thorn you can use Honey Locust, you know..... The tree with the thorns that go right through the bottom of your shoe? What a blast those are! There are a lot of those around the lake at a disc golf course I like to go to. Come to think of it, I know there's a few in my backyard.. If it was me, I'd try to find myself some of that rotting wood that is just falling apart, and get myself a piece of that and use it as the plug.. It's light, and easy to work with.. You should be able to push a Honey Locust thorn right through it.. Heh, or maybe even try part of a mushroom as a plug... Of course you would need to know it that thing is toxic or not.. Maybe just wrap some grass around the shaft and secure it with pine pitch to make a plug? Resembling the cotton plugs that the tribes of the Amazon use, only made of grass.. Also.... You're in the woods, you could probably find a lot of litter that was thrown out there that could be of use for dart making.I'm still trying to think what natural materials; if you looked hard enough might be able to find some acorn caps that would fit the bore and figure out attaching a shaft (?); now I gotta try it... Or maybe push a long thorn through an acorn nut?