Most NF websites have veribiage advising what to look for in terms of a safe place to shoot. I.e. away from people, earthbackstop, no trails cutting across the line of fire, etc. Basically, you "legally" can shoot anywhere you can determine it is safe.
It may not be a developed range that someone is providing to you for free, you might need to hoof it to find a safe place to shoot, a forest ranger may disagree that it is safe because they know of cabins, trails, backcountry campsites in your line of fire, but you "can" shoot in NFs.