Problem #6 is that the study came out of...

...the Johns Hopkins Bloomberg School of Public Health. Note the "B" word in there. They are another wholly subsidiary of Michael Bloomberg and any studies they released must be viewed through that light.

5 Problems with the New Study 'Proving' that More Background Checks Lowered Connecticut's Gun Murder Rate by 40 Percent

To summarize it. They cooked the books. Surprised?