While researching an entirely different subject, I came across some information which surprised me and I thought I would pass along:

The German firm of Keller+Simmann (http://www.einstecklauf.de) is offering small caliber (.22 Hornet)insert barrels to be used in large bore (9.3mm and larger) rifle barrels for practice purposes.

I wonder if any of our participants has had experience with such an arrangement.