When I tried to fit the new carbine barrel from Green Mountain to my Lyman Trade rifle (since their chart says it is going to fit) I noticed that the hook on the breech needs to be thinned. NOT A LOT MIND YOU. I could almost get the barrel hookinto the tang. I think a few strokes with a good file would have done the trick. Like a stroke on each side. As for the thimble rib that does have to be reduced. Depending on the rifle you are going to put the barrel on. I measured mine out for the T/C Hawkins, took a Dremel tool with a metal cutting disc and very carefully cut the thimble rib off. Then I have a blueing pen from Birchwood Casey for touch ups and just touched that up. It then fit like a dream.