The reason that the "middle ground" keeps getting re-defined is because America is a representative republic. And gun owners who want an unrestricted interpretation of the 2nd amendment only make up a portion of the entire republic. The rest of the republic also gets represented in these debates.
The question about criminals is a good one. But it's not really the spark that has lit the current debate. The current debate is centered around keeping guns out of the hands of troubled young people, minimizing the amount of damage they can do, and most of all protecting children in schools who have the right to life, liberty, and the pursuit of happiness. Adam Lanza wasn't a criminal until he pulled the trigger on his mother.
Please don't misunderstand me, I am in fervent support of the second amendment. But I believe our constitution is wise. That the state of our union is solid. That thousands of men and woman across the country have taken an oath to defend the constitution from enemies both foreign and domestic. I was one of those servants and I am aware of the difference between a lawful and unlawful order.
I simply don't see the practical application of a "well regulated" civilian militia in our modern age.
"...whenever any Form of Government becomes destructive of these ends..."
That is the practical application of a "well regulated" civilian militia in our modern age.
Also...
"...being necessary to the security of a free State."
Further
"...for the defense of themselves and the State."