So the real problem you're saying is that we should never try and define things because definitions themselves can always be changed?
This problem has already been solved in other areas. Look at drug laws. They cover designer drugs easily - which is the exact same problem as you describe here.
And also - it can be a meta category, and it's helpful to have that. We can define semi-automatic pretty well, and the law has ways to cover "equivalents". Your scare quotes don't change that.
No, what I'm saying is when you're reporting on, or worse making laws about, a subject you should understand the words you're using. It wouldn't kill the media (or people in Congress) to spend twenty minutes learning enough about firearms to avoid looking like complete idiots.
And yes, you can define semiautomatics pretty well. But semiautomatics will not be banned. The problem is you can't define "assault weapons" in a way that they don't cover all semiautomatics.
It's completely reasonable to put scare quotes around a nonsense term like "assault weapons". If you say that at your local range people will point at you and laugh.
That's why gun manufacturers were able to make a few cosmetic changes to turn illegal guns into legal guns the last time we had a (completely useless) "assault weapons" ban. The California "assault weapons" ban was even dumber - they listed models in the ban, making it trivial for manufacturers to sell essentially the same weapons by changing the model number.