Skepticism on Anti-Gun Studies

I’m a bit skeptical on a new study. The BBC says,

Researchers claim a new study provides some of the most compelling evidence yet for tighter gun controls in the US.
The team followed the consequences of the State of Missouri repealing its permit-to-purchase handgun law in 2007.
The law had required purchasers to be vetted by the local sheriff and to receive a licence before buying a gun.
Reporting soon in the Journal of Urban Health, the researchers will say that the repeal resulted in an immediate spike in gun violence and murders.
The study links the abandonment of the background check to an additional 60 or so murders occurring per year in Missouri between 2008 and 2012.

This seems a bit strange to me: gun buyers are still required to undergo a NICS check at the gun dealer (even if they don’t need to get a permit from the sheriff, who also presumably runs the buyer through NICS), so how would doing away with a duplicate check and permit from the local sheriff result in an increase in gun murders?

The team said it took account of changes that occurred in policing levels and incarceration rates, trends in burglaries, and statistically controlled for other possible confounding factors such as shifts in unemployment and poverty.
The team counted a doubling of handguns shortly after sale being recovered from scenes of crimes or from criminals.

Interesting, though I wonder how they define “shortly after sale”. The article does not mention if the handguns being traced were originally purchased in Missouri or brought in from other states. Also, it doesn’t mention how they were able to get access to trace data in the first place, what with the Tiahrt Amendment still being in effect.
According to the article, the study was conducted by “Prof Daniel Webster, director of the Johns Hopkins Center for Gun Policy and Research.” I note that they forgot a key part of the research center’s name: the proper name is the “Johns Hopkins Bloomberg Center for Gun Policy and Research”.
Being a scientist myself I’m a big fan of science, but I’m skeptical of research produced by think-tanks, particularly those with major funding from a heavily-biased source.
Either way, the study is irrelevant: owning a gun is a Constitutionally-protected right. Requiring a permit or license to exercise any right, even if such permit or license is routinely granted, is wrong, period.

America currently has more than 300 million handguns in circulation. But the issue of gun control remains a hugely contentious one.

I was under the impression that the US had somewhat more than 300 million guns of all types, not just handguns. This was probably just an editing error on the part of the BBC.