Guns and Goldilocks

The left wants to ban so-called AR-15 "military style assault weapons" (to use the arbitrary PC term designed to scare fellow hoplophobes).  Here is mine.

I pose the following dilemma for the left:

1. Either so-called assault weapons in the hands of many citizens would have the desired effect of making the government seriously think twice about tyranny or not. 
2. If it would, then the legality of so-called assault weapons is justified for putting a check on tyranny.
3. If it would not, then there is no justification for banning so-called assault weapons (without also banning many other guns.)
4. So either way the legality of so-called assault weapons is justified.

What the left needs is that these so-called assault weapons are just right.  They are too lethal for self-defense but not lethal enough in the hands of many law abiding citizens to thwart tyrannical aggression.  But if they are military style assault weapons, they sure sound like they'd have the desired effect of (e.g.) checking the FBI from going door to door and confiscating everyone's guns.  No doubt the U.S. military could obliterate all citizens with its massive military might, but no government set on being tyrannical over its citizens would do so.