Make the underlug that you'll dovetail to the barrel, the full width of the bottom flat. It does not need to be 'slotted out' - a simple round hole to fit the screw is fine. this will keep the swivel from moving around and wallooning out the stock, which has very little strength in that part of the forearm. I have cut pin hole slots to make them longitudinally larger, to allow for stock movement, but in our climate, there is none, so now I don't bother. In over a hundred guns, I've never had one of mine split from stock movement. But our humidity here is fairly consistent and low - winter and summer.
