I have no idea what Lord Tony is going on about, but it can all be solved with physics. You could determine the damage of a bullet by finding it's kinetic energy. It's mass and velocity are what determines how much energy it has. Though the shape of the bullet and it's material also determines if it goes straight through or bounces around and forgets stuff up.
Legos would probably not kill a person, it might do some damage. The fact that they have a super low mass and that they're velocity would be low cause of stuff aerodynamics.
On top of this a rubber bullet is shaped like a normal bullet, and fires out of a normal gun like a normal bullet. It pretty much is a normal bullet except for the fact it's made of rubber. If you were to have a plastic bullet it could very well fall apart making it pretty much no lethal.
But the issue of a plastic gun is that all the energy shooting a bullet creates would pretty much destroy it. Seeing as how the receiver of the gun pretty much fell apart, and that really doesn't even have to stand up to that much energy.
In short. Physics motherforgeter, use it.
