It's all down to the water flow through the pipe. Low flow and the water has more time to absorb more copper, and in time can get way past the legal limit. Domestic flow is usually high enough not to cause a problem. However it might be a good idea to flush your drinking water after long periods of no demand.
Now talking of toxic heavy metals, what about tungsten, nasty stuff, carcinogenic etc. Perhaps not to Duck, but to the shooter??? What about plastic wads, I'm old enough to remember the first plastic wads and the manufacturers feeding them to live stock apparently without ill effect.
Is this shooting's death by a thousand cuts?
Sorry about the rant !