I thought Air Marshalls all used frangible ammunition to prevent decompressing the cabin should they have to shoot while the plane is in flight? Do they really use plain old HP's?
I don't know one way or another what they use.... However frangibles thus far only work on things like walls and trees. The soda-can-like skin of an airplane would get punched by pretty much anything.
Thats why I think the .357 Sig is an interesting choice. It has alot of power, however should it punch the airplane, it'll make a slightly smaller hole than a .45, or even the .40.