At the very least this sounds like conspiracy to violate the weapons laws of the District of Columbia. Further, it shows that they knew that their plans would violate those laws as they were concerned not to violate them until the very last minute.
Ultimately, I suppose, it comes down to what the status of private armies is in the US. I’m not sure if there are active duty military or law enforcement who are members of the Oath Keepers, but if there are they should be rooted out of the service. A man cannot serve two masters.