As you notice, most inserts are indeed designed for the right barrel so that you can take advantage of the set trigger. As a result most builders regulated the left, and usually tightly choked tube, to still shoot the Brenneke design with some degree of accuracy. However, these were never designed to be hundred yard weapons - that is why the rifle is there. But many drillings will put that slug into pie plate accuracy at fifty yards which is/was minute of boar on a drive hunt where the slug would make sense.
