Has anyone tried these extensions?

Create a problem, to sell the solution.

18th century tech works alright lol. Also the solution adds another thread, three pieces instead of two all up and tolerance stacking is something to think about.
 
I'm calling bullshit. All you're doing is adding additional room for error when chambering a barrel.

Any decent Smith will indicate off the barrel for straightness.

This "new and improved" chamber addition does nothing because when they thread the barrel to fit the adapter it is no different than if they were to just cut a chamber in the barrel.