On those occasions when I find it impractical to to be armed with a large sidearm, I often carry a Beretta 950 in .25 calibre.
Since I have expended sufficient time and effort to consistently hit the head at seven yards, I feel comfortable with this little gun. The Hornady XTP ammunition that I load it with registers 950 FPS on a chronograph. Speer's Gold Dot ammunition is only slightly less powerful at 875 FPS.
In my opinion, it fits in well with the three basic rules of self defense.
1. Avoid trouble if possible
2. Have a weapon and be prepared to use it if necessary.
3. Possess the necessary skill to use it effectively.
Obviously, should it become necessary to actually engage another human being, I would much prefer to be armed with a very powerful sidearm like my 9MM SIG-Sauer. Unfortunately, that is often not practical.
The soldier's pack is not so heavy a burden as the prisoner's chains. Dwight Eisenhower