There is an increasing market for frangible bullets — ones which will stop someone, but not create a blow-through or riccochet hazard. Immediate possible application: arming air marshals.
Our bullet can penetrate a bad guy’s head, but it won’t come out the other side.
   — Dan Smith, Director of Technology, SinterFire
(Via Sonic Blog)
I dunno. They don’t come out because they absolutely kill whomever they hit. How hot are these marshalls with pistols? Maybe rubber bullets would be better. Short range and virtually no chance of smuggling body armor aboard… And all you really have to do is soften them up for the passengers.