Anyone that has flown post 9/11 knows that airport security has greatly tightened. However, there are still worries that bad guys might slip some weapons past the security at the airport that could be used to take over an aircraft. The invisible weapons that Sprave is testing on his ballistic gel are not literally invisible; a metal detector just won't detect them.
Among the weapons are some knives made from things like ceramic and flint along with one of Sprave's slingshots made from carbon fiber. It never ceases to amaze me just how powerful a slingshot can be. Ballistic gelatin is the stuff that mimics the resistance of flesh so it's interesting to see just how bad the gel is damaged by this stuff.
Most of the knives slice and stab just as you would expect. The stones that he is flinging from that carbon fiber slingshot are very large. Despite how large they are the stones still penetrate the gel and lodge inside. The smaller stones with sharper edges penetrate surprisingly deep into the gel. That slingshot could certainly kill someone.