Driving under the influence is an example of how things could work. I know that driving is a privilege, so no need to educate me---this is just an example of how laws and regulations can be effective.
When I was going to college in Colorado in the late 60's there were often class mates that were arrested for DUI. A good lawyer would often get them off with little consequence, and even if tried and convicted, the first offense amounted to a couple of months having no drivers license and a small fine. Fast forward to the present. Now a person that gets a DUI is in for a lot of money and consequence. Last I saw it cost about $10,000 to get your license reinstated, buy insurance, and there is often jail time, convicted offenders are required to attend AA meetings and do some significant time performing community services.
http://www.alcoholalert.com/drunk-driving-statistics-colorado.html
In 1982 there were 422 fatalities that involved drunk drivers, and in 2014 there were 187 fatalities. Needles to say the total number of drivers has increased a lot over that time. Other things certainly factor in, but it would be ludicrous to pretend that tougher laws and stiffer consequences had no effect.
The two points are:
1. The tougher regulations were very effective
2. There are still offenders that get by, and still drinkers that kill people while driving.
No laws or regulations are perfect. In my mind, trotting out the argument that this shooter could have still acquired his weapons is only a method of detracting from meaningful dialogue about how to
do better at keeping guns out of the hands of killers.