First of all, I doubt TSA agents are "well trained". They are incompetent at their main task, which is screening for weapons in luggages and they are assisted by state of the art technologies.

Second, I also doubt people waiting in lines at the airport will be anything but rational the moment someone take out a gun and or a bomb. At least on a plane, nobody can run anywhere. In the airport check in, there will be shoving and yelling and stepping on each other to get out faster.

Also, it is harder to smuggle a rifle onto a plane. It isn't difficult to bring something extremely destructive to the check in gate.