Sorry, that's not true. Assuming it is legal to carry a gun, which of course it wouldn't be in many other jurisdictions, firing it in self-defence would be a valid defence to a murder charge, even against an unarmed opponent. Again. this is not a Florida thing.
Taking the Tony Martin case in the UK, for example (man who was repeatedly burgled and eventually lay in wait for the burglars to come and shot them with a legally owned shotgun when he did), he was convicted for murder because he a. planned it (though this later got appealed out on grounds of diminished responsibility) and b. because the burglars ran away.
If, however, one of them had attacked him and he'd shot one dead, he would have walked. Of course that's very unlikely, as he would have had to retrieve the gun from its securely locked cabinet you have to keep it in in the UK, which makes a spontaneous self-defence plea difficult, but the fact of the matter is that it doesn't matter what tool you use for self-defence.
Whereas if you use, say, a knife, you can still be done for murder in the UK if you strike repeatedly; a single stab to defend yourself is fine, but anything beyond that (unless you can establish that your life was still in danger after the first stab) and you are in trouble.
It's never the implement that matters, but the circumstances.
I'm really not sure people are fully clued up on what manslaughter is here. A self-defence plea invalidates a manslaughter charge as much as a murder charge. It would make no difference. Manslaughter is not some sort of simple 'we can't get him on murder so we'll get him on this instead' magic button. It always involves an illegal act, and a self-defence plea, by definition, is a claim that a person did nothing illegal.