.357 SIG Over Penetration

Question
With all the research I have recently completed on the Speer Gold
Dot .357 SIG round, and its reputation for being able to
successfully and continuously penetrate barriers such as
windshields and metal car doors, do you feel this is the most
appropriate and safe ammunition for the US Federal Air
Marshals to be carrying on airplanes?

Answer
In my humble opinion, the .357 SIG has very few self-defense or law enforcement applications.  The problem with the 9mm's ineffectiveness (depending on ammunition used and whom you speak with)boils down, often, to its overpenetration, this due to relatively small diameter and rapid velocity.  To then take a 9mm projectile and drive it faster (as the h.357 SIG does) does not make a lot of sense (at least there should have been a clear question that the development of the round was seeking to answer).  A frequent problem with 9mm ammuntion is collateral damage due to overpenetration.  The only reason it is not as prevalent with the .357 SIG is that the cartridge is not as common as the 9mm.

Of the .357 SIG ammuntion available, the Speer Gold Dot is among the hotter offerings, thereby increasing the risk of overpenetration and collateral damage should the projectile fail to expand and lodge in fleshy medium.

Asked for my opinion, my recommendation for the USFAM program would be the .40 S&W loaded with either MagSafe or Glaser Safety Slugs.