There are reasons for doing a rimfire conversion either way.
An offset chamber would be used when it is desirable to not alter the action. This method requires either an oversize barrel blank so the outside can be turned eccentric from the bore, or a large enough bore to install a liner off center, such as a .22 in a shotgun.
Moving the firing pin location in the action can be simple or very complicated depending on the action.
It boils down to which method is easier for that action and the desired result.
I have done several rimfire conversions. One is a Winchester Hiwall takedown. It has two barrels, one .45-70 and the other, .22 Long rifle. The rimfire barrel has an offset chamber with the muzzle concentric. There is an extractor mounted in the barrel that looks much like a Ballard rimfire extractor that is actuated by the .45-70 extractor. This project was done to provide two barrels of identical weight, one for Silhouette, and the other for practice. The rimfire barrel started out as a 1.5" diameter blank.
I also have done a number of rimfire conversions on the Ruger#1. For these I move the firing pin and reline the existing barrel. This way I end up with a rifle that looks the same as when I started, just a different caliber.
