It has been interesting reading this thread, especially some of the suggestions regarding impromptu suppressors for a firearm. Here are some of my thoughts on the scenario that seems to be under consideration. That is a unanticipated opportunity for the woman to kill a dangerous individual in a men's room in a restaurant.

Here are some of my thoughts.

A .22 caliber handgun would be the way to go. Such a gun is easy to conceal and so would be likely to be something the woman would have on here at all times. Larger calibers would be more effective, but also produce more noise. To have assurance of being effective a head shot would be required, ideally to the base of the rear of the skull assuming that she can thus approach him from behind.

While the various suggestions regarding homemade silencers are interesting, and many of them may work, none seem to be the sort of thing that the woman would just happen to have on her and be able to quickly attach to the gun If one is assume that a silencer is needed it would make more sense that she would always carry a manufactured suppressor and a handgun with a threaded barrel to attach it.

Regarding the choice of a revolver versus a semiautomatic pistol, one thing to consider is that if the gun is to be held close to the individual to be shot it is probable that blow-back of blood and tissue from the first shot will jam the operation of a semiautomatic, this is far less a potential problem with a revolver. While a single shot head shot at close range with a .22 in the proper location is likely to fatal, but better still would be multiple shots.

In the scenario described the advantage of being isolated with just the woman and the man in the restroom is that she can shoot the individual without anyone noticing immediately. In my opinion though her immediate actions after that should be to quickly ditch the gun, say in the waste container, clean any blood off herself, and quickly and quietly exit the restaurant. One disadvantage of the men's room location is that someone else could unexpectedly enter at any time. This of course assumes that the gun can not be traced to her.