"As for the cutting a machine gun barrel well, I’m not entirely convinced that it could not be done."
Yes, it could be done, but you would have to use the same method that Buck used to cut through a bolt. Place the machine gun barrel on a solid surface, place the edge of the blade on the barrel and tap the back of the blade with a hammer. If you have a good blade, you should be able to eventually cut through the barrel. The blade will be damaged. BUT, that's not the story. The story goes that the barrel was cut with one slash of the sword blade. It ain't gonna happen.
There is a reason why these tales are called stories. They are just that, stories. Believe what you want and don't forget Santa Clause, The Easter Bunny and the Tooth Fairy.
Japanese sword blades varied from very good to very poor. Just because a blade is 500 years old does not make it good. I doubt that even the best of them would fair very well against modern tool steels.