Well I am definitely knowledgeable about firearms and I agree with HALOJUMPER, there exists this urban legend/myth about caliber performance and as I stated previously, the science on its face would suggest that a larger heavier projectile would provide better performance than a lighter smaller one, however in practical application in the real world this is not the case necessarily. As stated, it is all about shot placement first and foremost.
Points in case,
1. years ago I read a WWII story about a US Marine who was shot something like 10+ times with a stolen Thompson submachine gun in the Pacific and lived; that was 45 ACP 230 grain ball.
2. there exists some old footage of a cop in a large US city shooting a person with a 1911 (also .45 ACP), the person rolls over a car fender out of view, when the cop comes around the corner of the car the guy gets up and runs away (albeit bleeding).
3. During the Korean conflict the US Army sent in a team to research equipment to determine what was working and what was needed. On the issue of guns (everyone had M1 Garands in 30-06 then) the #1 complaint was the inability to kill the enemy. Upon further research it was discovered that in many cases the soldiers were not actually hitting the N. Korean or Chinese soldiers or were only wounding them, but they continued to fight. Keep in mind that at the end of WWII the Army pretty much did away with real marksmanship training.
I could on and on as I have a lot of experience concerning this issue and have been involved in a lot of research, training and application of this. More people are killed with .22 than anything else. Many people have gone online to talk about how terrible 5.56 is, yet many bad guys would tell you that it kills just like anything else.
9mm does not suck, many people have been killed with it. When I came into the Army we were still carrying 1911's with 7 rounds of 230 grain .45 ACP. When we went to the M9 in 9mm, everyone was all pissed off, however we quickly determined that it was exceptionally capable and since it was a secondary weapon system, it was not one we started the gun fight with, but if you got into a situation where you needed it, I would rather have 15rds of 9mm than 7 of .45 (there are a lot more bad guys than I have bullets). That said, for personal defense, carry what you train and can hit with, the caliber is not nearly as important as people would want you to believe.
If you want to carry around a hand cannon in .44 Mag, go on with your bad self but when that gun recoils and you are way off target and cannot reacquire quick enough, at least you will know you are the one with the biggest gun there when the dumbass banger puts 15 rounds of 9mm on you. There is a lot more to carrying a weapon than just having it, you need to be able to conceal it, draw it, present it and hit a specific spot with it (perhaps more than once as demonstrated by the video that started all of this).
Whatever you carry you are betting your life on, if thats a .45, 9mm, .44 mag, or .22, make sure you can use it correctly. There are 3 things every man is absolutely positive they are great at; screwing, driving and shooting... most are lucky if they can do one of them because all of them take practice..