The researchers themselves are arguing over the best way to research the issues;
http://www.theguardian.com/us-news/2016/mar/10/gun-control-study-flawed-researchers
"Implementing three state gun control laws at the federal level could reduce the rate of American gun deaths by more than 90%, a new study has found.
But leading gun violence researchers have called that result “implausible”, and said the study’s design is so flawed that some of its findings are not believable.
The paper, published in the British medical journal The Lancet and written by researchers at Boston University, Columbia University and the University of Bern in Switzerland, found that one of the three most effective gun policies were laws requiring ballistic imaging or microstamping, which help law enforcement identify guns used in crimes.
Experts noted that the laws, which were on the books in only three states, were not actually being implemented in practice."
That suits the pro gun lobby to a tee as it means the status quo is preserved.