I did one a few years ago. I couldn't find my original post (if there was one..), but here is a link to a similar question from some time ago. I posted a few pictures of how I did it. It was my first time, and it worked out pretty well.
https://americanlongrifles.org/forum/index.php?topic=40390.msg391768#msg391768What kind of nose cap are you doing? A solid one? or a chevron one like on southern guns? That makes a difference.
The short is, you need to make a dam to hold back the pewter from running down the stock and the ramrod channel. I made my dam out of manila file folder card stock. it worked pretty well. Tape it up with blue painter's tape.
Make sure the seal is good, and make double sure that you plug the ramrod channel with something. If you get pewter down the rr channel, you are in a bit of trouble.
If it is a solid nose cap, relieve wood around the fore-end like you would for a brass cap. Doesn't need to be super thick, but if it is too thin, the pewter will harden part way through your pour and you won't get good results. I did that on a chevron nose cap.
Also, if it is a solid nose cap, consider drilling a hole on either side, into the two angled bottom flats of the barrel channel. Then countersink them slightly from the inside. The pewter will flow into the holes and act as a rivet. You need to have the barrel in good and tight when you pour, so the pewter doesn't run down the barrel channel. Some guys pre-heat the barrel to help stop the pewter from solidifying early.
If it's a full stock rifle, plug the muzzle of the barrel as well in case you miss! Don't want pewter in the barrel.
Your pewter should be hot and melted, but not so hot that it burns a toothpick in less than a few seconds. In other words, heat it up, and then dip a toothpick in it for a couple seconds. If it comes out all burned up, you are too hot.
I've done two of them, and this is all from memory from a few years ago. I think I've hit the high points.
Cheers,
Norm