In his picture 4 of 4, look past the cylinder at the rear of the barrel. There is twice as much steel there (around the hole) as on my wife’s model 66-3 357 mag.

That barrel is probably fine for a six shooter but much to thick for a 10 shooter (in my opinion). Without seeing one up close I guess the barrel covers at least one third of the end of chamber of neighboring round.

When the blast comes back off of that into the neighboring chambers it is no doubt hot. Need to put an empty fired shell with a little gun powder in it, in one chamber. Put a 22 long rifle next to it, and fire it, to see if it ignites the powder. If it does-not then that ends any of my speculation about flashover.

(But with the powder from a 22 rimfire burning for about 16 to 18 inches of rifle barrel - I think I am right).