Might be a good idea to read the Pre-emptive strike thread as well (
http://www.martialtalk.com/forum/showthread.php?97284-Pre-emptive-strike ), looking especially at points of law. A MA is always going to be held to a higher standard. We are "karate experts," remember? We are weapons. Yessir.
Many US States allow use of force greater than that being defended against, if you can show you are the defender, and are at a disadvantage because of your attacker's greater presumed ability, and had no path of retreat. Remember, you may be required to prove that in a court of law. Those deciding your fate may never have been in a fight. Regardless, they are sitting comfortably in a jury box or behind a judge's bench, taking a lot of time to decide what you probably had seconds or split seconds to decide.
Now, what weapon are we talking about? A man takes a swing at you and you reply with a 12 gauge shotgun. That's going to be a tough sell in a court. Not perhaps impossible, but very very difficult. If he takes a swing and you use a baton to apply a come-along hold. I see no problem. A lot will depend on the weapon. If you pull out a roll of quarters and hit someone, you will likely have some explaining to do. If you happen to have a cell phone in your hand and use that, that will probably be seen as a matter of circumstance.
Just remember that anything we say here doesn't apply when it gets to the stage of you going to court to defend your actions, whether criminal or civil. You will then be alone and have to convince the fact finders in the court.
But I really wonder about a belted MA thinking of using a weapon first. What have you been taught? Mind you I am not against weapons per se. And for sure if you think you need them to survive, survive and fight any court battles as a survivor later. But aren't we susposed to be a little better than that?