For some time, the US Army Corps of Engineers (USACE) refused to allow the carry of weapons for self defense, even if the person involved held a license or permit. Even in recent times, as more and more states issued licenses, the USACE refused to honor them, despite the fact that they were recognized in State Parks, and in National Parks that are not run by the USACE.
For those wondering (and I did) the USACE runs State Parks and National Parks that involve dammed waterways, and they maintain the dams. This sets these parks apart somewhat, in that they are managed by a branch of the military, rather than by the civilian parks departments. Because of this, a lot of the park rules reflect those in effect on Army bases, and one of those rules has been a prohibition on carry of weapons, even by service personnel.
As the other park systems began to relax their prohibitions in the wake of less restrictive carry laws, the Corps remained mired in the past. But state groups, notably GeorgiaCarry.org, continued to press the Corps to allow lawful carry by license holders.
In 2019, following a series of lawsuits by GCO, the Corps agreed to issue permission letters, if a citizen were to write and ask for such a letter. Then, should one be asked, this letter could be presented as proof of the right. But to be honest, even this restriction is too much, and GCO continued its legal pursuit.
And finally, this year the Corps agreed to change the rule! The proposed rule change is open to comments, and I highly recommend you go here and read the docket, and submit your supporting note. The deadline to submit is June 12, 2020.
Look for updates here as the rule gets closer to change.