The legislature is supposed to be a deliberative body, which applies careful study to an issue, such as gun crimes and then makes policy through laws that will govern the behavior of millions of people. Yet, when it comes to legislation dealing with many criminal matters, it is created in a complete vacuum of substantive evidence.
Lawmakers often draft laws, literally, making things up out of whole cloth. The use of sex offender registries is one example. They were created based almost entirely on fear and speculation, and the last twenty years have shown they do little to reduce sex crimes while damaging those convicted sometimes to the point of suicide.
Similarly, the "Tough on Crime" movement of the last 40 years was based on the notion that crime was encouraged by "liberal" judges who were too lenient in their punishment, and what we really needed was draconian punishment that would lock criminals away and toss the key.
The legislatures of this century, like someone dealing with the effects of intoxication the morning after, have been reeling from the increased cost of prison systems that grew exponentially for decades.
And yet, after all that, there still is little evidence that demonstrates these laws, with all of their mandatory minimums, add-ons and enhancements actually have had anything to do with changes in crime rates.
For one, the rules tend to be complex and convoluted. It is unlikely that many engaged in criminal activity have visited the U.S. Sentencing Commissions website to calculate out what their potential sentence could be if they were apprehended and convicted.
Laws on the books too often are a combination of wishful thinking and cover. Politicians are hopeful the laws will do something, and even if they have no effect, they can advertise during the next election cycle that they passed "tough" laws to protect their constituents from crime.
Source: themarshallproject.org, "Politicians Still Say Longer Prison Sentences Prevent Gun Violence — But Do They?" Dana Goldstein, October 14, 2015