Although that one does look quite bad, it is very common......I've seen the top hat of the liners cracked away......My guess is that whilst apart, most don't look at the height difference between the liner hat and the recess in the head.......The liner should always be the sacrificial part not the head so much.......This is why on rebuilds I near always replace the barrels complete, as there are many items on these barrels that need carefully measuring......not just the bore.......I've wasted money getting barrels bored/honed only to find after that the liner height was wrong and would not seal in the head.......had to fit new liners and start again.......we've all done it.......Ideally the head recess should be 0.125" depth and the liner hat should be 0.127" or there abouts.......not always easy to achieve, but that is what the difference should be.