The study in the New England Journal of Medicine was observational in nature and stopped short of being able to prove any cause-and-effect.
But researchers hypothesize that the decline they witnessed was due to the "brief period of gun abstinence during such conventions."
The findings also counter the widely held belief that accidents and injuries are more common among people who are inexperienced with guns.
"Fewer people using guns means fewer gun injuries, which in some ways is not surprising," said senior author Anupam Jena, associate professor of health care policy at Harvard University Medical School.
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The study examined nearly 76 million medical insurance claims for emergency department visits and hospitalizations related to firearm injuries between 2007 and 2015.
Researchers compared how many gun injuries occurred during NRA convention dates to the number of injuries that took place on identical days in the three weeks before and the three weeks after.
"Gun injuries on nonconvention days occurred at a rate of 1.5 per 100,000 people, compared with 1.25 on convention dates -- a 20 per cent difference," said the report.
"The biggest reductions in injuries during convention dates were among men in states in the South and West that have the country's highest rates of gun ownership, and among individuals residing in the state hosting the convention, all of which support the notion that the reduction in injury rates is related to attendance at the conventions," said the report.
Researchers were unable to measure whether convention dates also coincided with a decline in gun-related deaths, since gun-related homicides, suicides and fatal accidents are relatively rare.
According to the US Centers for Disease Control and Prevention, there were more than 65,000 intentional firearm injuries in the United States and nearly 16,000 unintentional firearm injuries in 2014, the latest year studied.
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