That is simply rationalizing. Plenty of mass shootings have happened
with 9mms, usually pistols but your assault rifle would do just fine.
The "military style" thing is really a red herring. Your CZ is
certainly "military style" and they will sell you a tactical style
stock for just about anything. You can certainly make a very
"military" looking weapon out of a Mini14 or even a 10-22.