You only need to "grab" one politician for the time-machine trip: Secretary of State, Thomas Jefferson, who authorized--signed off on--the 2nd amendment, which reads:  "A well regulated Militia, being necessary to the security of a free State, the right of the people to keep and bear Arms, shall not be infringed." 

Jefferson would be stunned and horrified at the carnage today.  Unable to see the shooter--and hearing the shots fired--he would immediately believe there was a small group ("a militia) killing the concert-goers.  When told it was one man, he would be incredulous. When he saw the victims were unarmed, he would be enraged. A likely question he would ask, "What did these people do to deserve this?"  When answered, "nothing."  He would be rendered speechless. The shocked man might then mumble, "but it was for security! We clearly said, "well regulated..."

And finally, "Why hasn't the country amended the constitution on this issue?  Why?"