The Blaser R8 .22 rimfire conversion kit includes a barrel, breechblock, and magazine insert. It comes in a hard case that accommodates two barrels, two scopes, other bits and pieces, and tools.
The concept of converting a high-powered rifle to .22 rimfire, temporarily—for plinking, practice, and pest control—has a very long, very checkered history. Over the last 100 years, just about everything has been tried, from barrel inserts to chamber plugs.
Frankly, not many have worked out too well. They’re either not accurate, limit you to a single shot, or are hard to operate. There’s always something.