Liberty has been shown to be a cheap gimmick bullet that does minimal damage, and now its trying to come back as a possible armor piercing round, a result of the higher velocity low weight projectile. Its a poor choice for unarmored targets, and its effect even if it does pierce armor is questionable. If one were truly serious about the handgun selection for soft armor concerns, one would choose the 5.7 FN, its soft armor punch through is one of the reasons NATO was interested. I'm no big proponent of the cartridge, but being objective, its good at what it does. Small caliber, high velocity, it is perfect for its intended role. The Liberty as a compromise for armor piercing in a larger caliber is a poor compromise, either go with a standard HP in the standard chamberings, or just switch to a whole cartridge that is intended to do the job.
On top of that, if the attacker is wearing hard body armor, composite ceramic or steel, frangible high velocity rounds will go from potentially useful to worthless. The high velocity light bullets may pass through soft materials before they can resist to stop them, but are all but pointless against a hard object, which they will only fragment against.
The chance of an encounter where one has to face off against an attacker with armor is not impossible, but far less likely than unarmored. It wouldn't make that much sense to specialize for the least likely scenario, rather than specialize for the most likely scenario. Bullets good at piercing armor are seldom useful for anything else, and you are downgrading your overall performance for one situation.
I don't think the pelvic gurdle shot is the best idea, but then again, its also thinking on a right track. Head shots, in a stressful scenario, can be difficult to make, and if we are talking about an attacker wearing heavy body armor, why might he not wear a helmet that could help with many pistol caliber head shots? Perhaps the pelvis is not the perfect target, but it is unarmored and can be torn apart by rounds better than the armored part of the man. Most importantly, you are still staying on center mass, where you will likely hit in any case, and perhaps hit underneath the armor itself. Many armors, especially plates, are short cuirass style, and will often leave a significant part of the torso uncovered. Shots below this armor may mostly be low value intestines, but in many cases will end up with potential shots to the spine and descending aorta and vena cava, and in some shots the kidneys and renal artery. Less value than the upper chest, but still better than hitting armor, or a complete miss.
A man aiming for the pelvis might put a lucky shot through the attacker's spine or descending artery, especially if he pulls the shot. A shot to the head with a handgun may easily miss, will hit nothing if the shooter pulls the shot up, is not exactly a guaranteed end to the fight depending on where the shot hits, and presents a smaller target. Shots at the pelvis might also hit the unarmored legs, which may not stop the fight initially, but can decrease combat effectiveness, and in the case of the latest terrorist attack, provide wounds that could prevent a second attack elsewhere by making them a long term casualty, i.e. shot to the leg may not stop the first massacre, but may put the shooter out of the situation of a second mass shooting later on. I argue that if a terrorist is bold enough to wear body armor, they might wear head armor, and a lower torso shot is more likely to at least slow down the attacker.
No easy answers on this one. On one hand, this is theory tossed about, about a subject that almost no one will realize. On the other hand, the very nature of terrorism is changing to the point it may be more important than what we realize.
