I would think a .410 insert in a 12ga bbl would have enough wall to completely contain the pressure & put virtually no pressure at all on the "Carrier" bbl. In fact I would highly susoect a .410 tube having even the length of the old Savage .410'er could be fired through a smoothed out bamboo tube opened to full 12ga size without splitting it.


Miller/TN
I Didn't Say Everything I Said, Yogi Berra