Laws only affect the law-abiding. Disarming law-abiding people does not make us safer.
We do not know what threats will exist in 20, 50, 100, or 200 years. The fact is that semi-automatic guns exist. FULL-auto rifles exist.
I'll be damned if I allow the elected morons, and non-elected bureaucrats, to determine what's going to be the best guns my great-great-great grandkids will need, to defend against the terror of their time. Could be radicals from any part of the political or theological spectrums, and could be foreign or domestic. We just do not know, and since our forefathers saw fit to put "shall not be infringed" on paper, IN INK, that we ought to do our damnedest not to give in a single inch.