What do you mean by responsibly? As a responsible gun owner, I adhere to all the legal requirements of gun ownership, regardless of my personal political opinions about their validity.
In terms of technical usage, I am sure this guy is responsible enough to use a gun. It is just that the law says he needs to register them, and such laws are Constitutional in your country and mine.
The last thing I want is the government deciding for itself when it is going to enforce the laws we have democratically decided upon. That they managed to enforce them without doing serious harm to either the principles of justice or this guy is commendable, not something to be criticized.