Given the primacy of shot placement in any serious discussion of a bullet's lethality, I would guess that most bullets are not lethal.
I think your guess would be wrong. Do not confuse the lethality of a projectile with the ability of an individual to place it in a lethal spot. They are TOTALLY different matters.
One could say the majority of bullets fired do not result in fatalities, and that would be correct. But the bullets themselves have lethal capability regardless of whether or not you can use it.
ALso, I'd like to point out that there are no "less lethal" projectiles. I think you are "drinking the Kool Aid" if you use the term "less lethal". Its just BS.
Less LIKELY to be lethal, I get that. It's actually true. But remember, people have been killed with every kind of "less likely" round too. People have been killed by BLANKS! People have been killed with BB guns. TASERS can kill people (with heart conditions). There is no 'safe" non-lethal weapon, including hands, & feet.
All there are, are weapons that are less likely to cause death when used correctly. And this ball on the end of your pistol gadget seems like a cartoon made real. And when the ball caves in somebody's skull, when used in the intended manner, the maker better have some good insurance and good lawyers.
I'd like to point out another couple issues, particularly when considering private citizens using this device, or any other less likely to be lethal ammo...
First, in many jurisdictions, using a firearm, no matter WHAT you put in it, is legally using deadly force.
Second, using "less lethal" ammo, can be inferred that you did NOT feel deadly force was justified, and therefore, you are not justified in shooting.
For the police, it's a different legal matter, but for the private citizen, in a self defense situation, "less lethal" (correctly, "less likely to be lethal") does not seem to me to be a wise choice. Neither legally nor practically.