This doesn't challenge whether the statement is true, it challenges whether the statement is useful. If you think it could be made to be more useful you can use the "suggest edits" feature, or write a new statement. (It might be worth considering that if you successfully argue that a statement is too vague you haven't actually defended any position about gun laws.) It would also make sense to say what is too vague about it.
"People who commit mass shootings are not likely to follow any new gun laws, so new laws will not likely affect the yearly number of mass shootings. Most mass shootings and the overwhelming majority of gun homocides are committed with illegally procured handguns in urban areas that already have stringent gun laws. What new gun control laws would change this?"
It is NOT the case that "New gun laws will reduce mass shootings in ways other than preventing shooters from obtaining guns."
The statement above does not appear in any challenges, supports, or specifications of other statements.