If I May profer my opinion.

As She is a single shot anyway, consider having a 30 cal barrel turned up so that the outside of the chamber area is a perfect fit within the 16 G chamber -.002 +000 (take a cast first) Then pick a min barrel profile for the rest of the barrel.
Chamber it in something kind and low pressure like 30-30. At the rim cut put a small depression into which you can get a thin coin or your thumbnail
Now at the muzzle end of the liner leave a section like a thick washer on the last 3/8" and make this about .060 less than the inside of the 16 G muzzle. On this section turn a O ring groove so that when fitted it is a tight fit in the 16 G muzzle.
Now disable the extractor/elector or remove them.
Lube theinside of the 16G barrel with a bit of copper greese. Drive the adapter home with a rawhide mallet.
Shoot and enjoy and no harm done to a collectable.

Regards

If you cannot follow that then PM a fax number to me and I will do you a sketch.