Quick Rotella/Rodela durability question

[James Brazas]

Here's a quick question on Rotella/Rodela/Target durability.

George Silver recommends the Target shield as one of the best weapons to use on the battlefield due to it "leading upon the shot" and being able to better parry pole-arms and two-hand swords than bucklers can.

This seems to imply that steel targets were bullet-proof or at least bullet-resistant.

How bullet proof would a Target shield be? How heavy would it need to be to reliably deflect shots from the guns of the day?