Well actually "whether a gun is in proof" actually IS the issue. As Last Dollar wrote in his response in the Forcing cone thread noted by vabirddog, buyers and sellers may have differing opinions on the subject. Whether a gun is safe is an issue, absolutely, but not mine at the moment. I am still unsure whether lengthened forcing cones automatically moves a gun from "in proof" to "out of proof", safety and pressure issues aside.

The only constant in life is change.