The AGA was found switched on. And if you read point 30 in the Bamber v CCRC 2012 document. Boyce proved it was possible using an AGA cooker.
Aga's are usually always on. They take an age to heat up from cold.
How long does it take to heat up a rifle using an aga? Just because it's possible it doesn't mean that is what happened. You still have to have convincing evidence it was the end of the rifle and I'm not convinced.
Furthermore, if we accept Boyce's evidence, all it means is the silencer is off the rifle making the burns, not who is holding the rifle.