Bring in guns? From where
Just like drugs, humans, etc, guns are also trafficked. The vast majority of gun homicides are done with handguns and largely gang related. Not all of those guns weren't at one point legal guns in the US - where the gang member either stole them from a law abiding citizen or walked into a gun store. All handguns in the US require background checks and you have to be 21 years old to buy.
The thing I am objecting to is the notion that if an illegally used gun was used in a crime it must have once been a legally possessed gun, and (the implicit policy prescription) that the way to curb illegal gun possession is by limiting legal gun possession. While that last part probably tracks, because of my ideological beliefs, I'd rather we exhaust all other options before limiting legal gun ownership. And even then, again because ideology/convictions, curbing legal gun ownership is probably not a thought I'd entertain at the ballot box.