Below is a post from a long-time friend (which is similar, I’m sure, to many you have probably seen)… it is followed by my response, which may be helpful in changing the hearts and minds of the people you know with similar attitudes to his.
If the AR-15 assault rifle had been banned 5 years ago yesterdays school shooting would still have happened. A type of weapon is not the problem. We as a society are failing. But it's not because so called assault rifles are legal.
I am not "anti-gun"... I hunt, I shoot skeet, I shoot targets, and I enjoy it. I've shot any number of guns (including AR-15s) and expect to continue. But the problem with your premise is that with reasonable limits on ammo, and magazine size and number, FEWER kids would have died. What about the additional 1, 2, or 10 kids that otherwise would have survived this attack but for the wacko having an AR-15 with "countless magazines?"
Assault weapons are designed to fire rapidly so as to be better at killing PEOPLE. Reasonable regulation of who is legally-allowed to possess such weapons is appropriate. (See the first four words of the Second Amendment). But even if we don't limit the ownership of these weapons, and we continue to treat them as consumer products, those consumer products represent an unreasonable danger to society, and we need to make them safer by changing their design (e.g. fingerprint detection or microchip locking), limiting the amount of damage they can do in a given time (clip size/clip number), and/or strictly regulating their distribution (e.g. strict background check and registration). If you want a less-regulated firearm, buy a bolt-action. Expect that the more damage your gun can do, the more regulation you will be subject to... just like everything else on earth.