Secondly, we would not be searching for one "bad apple" (that can poison a dozen like in Texas), but for millions of: psychopaths, sociopaths, mass murderers, implicit murderers, and paranoid schizophrenics. The goal here is to make it harder to get to legal guns, thus increase the odds of being arrested while purchasing illegal ones.
Thirdly, refer to the second deviation from the intended communication. That there is the goal. Not have an entire US population round of for a crazy experiment just to have a research. Who would do such a thing! Why would you bring this scene up?
The reason I brought up research is because you asked for answers in an area that could be best researched after such a policy would first be put in place. You want more accurate numbers how more the odds would drop if fMRI is involved in background checkes, that research needs to be done after such policies - was my point. Not have a whole nation experimented upon!