Once a person is detained, placed in handcuffs and, in this case placed on the ground to further restrict their mobility (liberty), they are technically under arrest. The key here is that their freedom of movement has been taken away from them. Once that is done, they must be Mirandized before any questions are asked.
So here we have a case of a guy legally carrying a gun, being handcuffed, placed on the ground, denied his liberty being asked questions. What if he tells the cop he is invoking his 5th Amendment rights to silence and demands an attorney present during questioning? Does he go to jail? Is he charged with something? If so, what?
I understand that the police officers want to be safe while questioning someone. But there are better ways of disarming someone and checking them out.