There are 3 claims all mashed together here. Let's look at them:

1. There was a study that looked at Missouri murder rates in relation to gun control law changes.

   TRUE. [Repeal of Missouri's Background Check Law Associated with Increase in State's Murders][1].

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2. The study **claims** to have new evidence that **proves** ...

   Mostly TRUE. On one hand, the title study itself clearly states "associated". correlation!=causation and scientific evidence doesn't *prove* a theory, merely supports it.

   On the other hand, the study summary linked above quotes its author (full disclosure: he sounds like gun control proponent, not an independent researcher, from what I could find): 

   > "This study provides compelling confirmation that weaknesses in firearm laws **lead to deaths** from gun violence," said Daniel Webster, ScD, MPH, director of the Johns Hopkins Center for Gun Policy and Research and the study's lead author. "There is strong evidence to support the idea that the repeal of Missouri's handgun purchaser licensing law **contributed** to dozens of additional murders in Missouri each year since the law was changed."

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3. That the study actually proves its claims about causation:

   Possibly false.

   I will need to dig further into it, but one MAJOR red flag was already found by gun rights proponents:

   The study - while correlating Missouri murder rates vs. national - have **completely ignored** the fact that while murder rate in Missouri rose 17 percent relative to the rest of the U.S. in the five years after 2007, **it had actually increased by 32 percent during the previous five years** - which wasn't the case for national rate which was nearly flat([summary/research][2]), ([MI source][3]), ([US source][4]). In other words, Missouri:

   - Was already an outlier in national murder rate in 5 years preceding the law changes.

     ![enter image description here][5] [Image Source](http://crimepreventionresearchcenter.org/2014/02/what-does-missouri-show-about-the-benefits-from-universal-background-checks-the-forthcoming-journal-of-urban-health-study-by-the-bloomberg-school-of-public-health/)

   - Experienced a **slowdown** in the growth of murder rates after the laws relaxing gun control passed.


   Given that similar laws exist in several other states that were NOT included in this study - and that a cross-state study ([src][6]) showed a completely different result - it sounds like he cherry-picked one state that had non-gun-control-related murder upward trend.


  [1]: http://www.jhsph.edu/news/news-releases/2014/repeal-of-missouris-background-law-associated-with-increase-in-states-murders.html
  [2]: http://crimepreventionresearchcenter.org/2014/02/what-does-missouri-show-about-the-benefits-from-universal-background-checks-the-forthcoming-journal-of-urban-health-study-by-the-bloomberg-school-of-public-health/
  [3]: http://www.disastercenter.com/crime/mocrimn.htm
  [4]: http://www.disastercenter.com/crime/uscrime.htm
  [5]: https://i.sstatic.net/D691p.png
  [6]: http://www.amazon.com/gp/product/0226493660/ref=nosim/?tag=johnrlotttrip-20