Air Marshalls most definitely do not carry or use frangible ammunition. Most use some sort of a hollow-point though, which many agencies have moved away from based on more recent wound ballistics studies.

Putting a bullet sized hole in the hull of an airliner is not much of a problem. The training that most folks carrying guns on airplanes receive can be summed up as this, shoot as much as you need to, just don't miss.

Interesting statistics there Andy, again illustrating that we first need to get a handle on the problem, which doesn't seem to be the existence and ownership of semi-automatic and automatic rifles.