When the officer tells you to drop the gun, it doesn't matter if it's plastic-plastic or polymer-plastic, you drop it.
Because the cops will have no alternative but to shoot. Sad, but that's a fact of life. You can agree or not if the police have the right to shoot someone with a gun, but the fact is their job entails shooting people who pose a threat to others and it doesn't matter if it's a bright pink super soaker, they can't be assured it's not real.