Almost any handgun is underpowered. It's a tradeoff to find something that you can shoot (two-handed, strong-handed, and weak-handed), recover and shoot again.
The laws of physics say that a bullet cannot exert more force on your target than was exerted on your hand(s) when the shot was fired. So, a handgun bullet really doesn't have enough power to "knock someone down" without doing the same to the person that pulled the trigger.
Setting aside fragmentation and expansion (to avoid over-penetration), a bullet really isn't much more than the extension of a spear. The best you can hope for is sufficient internal injury to a vital organ to incapacitate someone quickly -- the central nervous system (brain, spine), circulatory (heart), or structural (pelvis).
There have been some studies that statistically analyze the use of particular calibers by police officers, but so many of them are confined to particular calibers on-duty that I'm not sure there is a sufficiently wide spread of samples to make a judgement.