I've poured hundreds of nose caps for half stock rifles, and have never heated the barrel first, though it is likely a good idea. I don't have pewter though - I use high speed babbit material, and I don't know its metal compound ingredients. But it pours well, fills the mold, shapes up nicely, and polishes.
You have to cut away some of the forend so that you can replace it with metal. Cut the sides of the forend right down to the barrel , and up from the bottom, until you have removed the entire rod channel and then a little more. There is quite a thin shell of wood remaining. Wrap the forend aft of the end of the intended nose piece with three layers of masking tape, This will give you a casting that is three layers of tape bigger than the wood. I make a mold out of cereal box card stock, since I never have beer case material at hand, so that it extends well up the barrel - perhaps twice the length of the intended cap, and tape it to the masking tape you already have.. I have used a putty formed of grease and asbestos (I know, I know) where steel wool is a better idea, to close the gap between the top of the forend and the junction of the barrel. You want a mold that has no place where the molten metal can run away. I plug the entry pipe with a short length of hickory rod that has several wraps of masking tape to make a tight seal, and yet will pull out of the pipe fairly easily once the metal is poured and hard. Sometimes I have to file a notch in the plug so that I can tap it toward the muzzle with a drift and a hammer, because the molten metal will fill every gap that you do not. Like Brian, I drill holes through the wood into the barrel channel and put a little countersink on the inside of the channel. I also drill into the end grain of the forend to further strengthen the cap.
Stand the gun up on its butt plate, heat the metal until it is molten and then some, clean away the dross, and pour it into the mold. Fill the mold to the top, so that if a void wants to form there will be weight of molten metal to close the void. A void can be caused by something as simple as gases given off from the scorching wood.
The metal will solidify quickly. But it'll be a while before it's cool enough to handle and work. But soon you'll be able to remove the paper mold, the little dams, and the masking tape. Make sure there is no metal above the side flats of the barrel...cut them off with a chisel if that occurs, because now it's time to take the barrel out. Hold the gun by the barrel in the vise, and tap out the wooden plug that filled the entry pipe, then take the rifle, and assuming you've removed the pins or wedges that secure it, give the barrel a good rap against the bench, and it'll lift right out of the forend. Now it's only a matter of shaping the nose piece to suit your idea of what it is supposed to look like.
It's been over twenty years since I did one, so I hope I haven't forgotten anything. I'll see if I can find a picture or two of a poured nose piece on a half stocked rifle.
