I see that a company in Germany makes insert barrels to convert modern shotgun barrels to double rifles. I believe the name of this company is Keppeler. Does anyone make similar insert barrels in the United States?

Some of these insert calibers seem to have really high pressures (I.e. a .243 win @ 65,000 psi). Are the actions in modern guns really that strong for the pressure?

If so I'd like to do an over/under in .375 or .470!