"On the mainland this year, there have been 221 mass shootings of more than four people at a time"
But, How many of those are from a single city ? Chicago ? How about in  Washington, D.C., New York, New Jersey and Massachusetts ?
What is the distribution of those 221 ?
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"It would upset the current system of gun regulation" - No, not exactly. It would create a single uniform nationwide law that everyone understands.
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"So far this year, there have been 36,497 gunshot injuries or deaths." 
Yes, and 2,212 of those are in the city of Chicago itself.
http://heyjackass.com/
and More than a dozen cities have higher rates of shootings and homicides.
https://www.thetrace.org/2016/10/chicago-gun-violence-per-capita-rate/
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Sorry, it is NOT just a fact that there is less gun violence here because of strict laws. the attitude of people here is completely different as well, and that single factor is probably the most significant factor.