I have just read a gun test on the Cobra by Geoffrey Boxall in Airgunner (July 1985).
Gerald Cardew also comments on the Cobra barrels and from what he says it is very clear that he himself had nothing to do making the barrels.
He may have done some for MAG but that was a seperate project.
I have pics of the test but absolutely no idea how to upload them. If anyone else can I can email or whatsapp them to you.
M.