It's not a problem.

The guys that hunt up there know where the plinking spots are, so they stay away from there, and believe it or, so do the animals

The few larger gravel pits are usually shared between people, but there are spots off the road that are "one party" spots and that pretty well respected up there.

It's a national forest, you can do anything you want on it, as long as you don't burn it down, or dump garbage.

Other then that, if you don't see a sign, you can do it.