Hi, Walter!

Thanks for you gentlemanly post.
In the interest of brevity and succinctness, I have edited it so that I might respectfully take issue with the substance of your comments.

This old saw has been circulating around the gun community for so long that people seem to accept is as gospel.

The actual fact of the matter is, if someone negligently shoots another person, modified gun or not, the shooter is, and should be, liable. (Just as if a person negligently ran over someone with their car.)
The only way a modified gun could damage the defendants case would be if the defendant fraudulently asserted that the gun went off without his control (i.e. that the gun was "defective").

IMHO that lie would be almost as bad as negligently shooting someone.

John