I once had a Kreighoff 22LR insert for a 16ga shot bbl. This one had a locating pin & had to have a notch cut in the extractor for it. There were then movable pads which allowed it to be sighted in to the gun's sights. It was locked in or removed by a sq key with a positive stop set & could be removed & re-installed without losing zero. It was quite accurate.


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