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Using statistical models to compare groups of counties, the researchers found strong firearm laws in a state were associated with lower rates of firearm homicide. Conversely, counties in states with weak gun laws had the highest rates of firearm homicide.

They also discovered that counties in states with weaker gun laws had lower rates of firearm homicide when surrounding states had strong gun laws. This suggests that when a state strengthens its firearms laws, both that state and its neighbors could see protective benefits.

"We thought that because guns can cross state lines, counties in states with restrictive laws might have higher homicide rates if they were near other states with more lenient laws," Kaufman said. "But what we found was somewhat the opposite. Counties in states with weak laws had fewer deaths than expected when surrounding state laws were strong.

"We can't say what causes this relationship," she continued, "but it is encouraging to think that these policies might have benefits that extend across state lines -- a bit of a halo effect."

https://www.cbsnews.com/news/gun-control-laws-state-impact-on-shooting-deaths-suicide-study/