Guys,
Thanks for the questions, I guess I need to clarify. The screw for the sling swivel does pass through a barrel lug, and acts as one of the barrel attachment pins. I do make the lug for the sling swivel a good bit more robust than a regular pin lug. I do like the idea of the gun being carried by a lug that is securely attached to the barrel, and not just to the stock wood. I am sure that you understand that the tiny amount of wood in a slender fore-stock may be damaged by the force of the gun being carried by the sling swivel.
The screw I use for the sling swivels is 0.139-34 size (18th c threads) and I make it from steel and not iron for strength. Only the very end of the screw is threaded where it enters the swivel, about 1/8 inch, the rest being a full 0.139 diameter cylinder pin.
Jim