I'm thinking about making my own horn and want to know two things:
Good for you. It's a fun thing to do and it can give you some feeling of accomplishment.
1) besides the brad nails how are the plugs sealed to the horn?
I heat the base of the horn with a heat gun (you can also use hot oil). You want the horn to be about 325 degrees so it will become somewhat soft. If you are going to use a lathe turned baseplug, then the best thing to do is to use a shaping cone to make the base totally round. Push the horn onto the cone until the horn is contacting the cone completely all the way around. Then turn the baseplug so that it is just a little bit oversize. Reheat the horn and tap the plug into place. If you are a bit leary about the seal this has given you, a small bit of beeswax around the inner lip of the horn before putting the plug in will further help to seal it.
2) what is used to seal the wood? I can't imagine that you'd use an oil since this will be what holds your powder in???
After I stain the baseplug, I apply a coat of linseed oil to the plug.
A couple of other thoughts for you. In real estate the 3 key words are location, location, location. In horn making the 3 key words are selection, selection, selection. You cannot make a handsome powder horn out of an ugly cow horn. Avoid excessive curves, avoid thick, heavy horns, avoid horns that have obvious flaws. If you are going to make a horn with a lathe turned baseplug, get a horn that is naturally close to round. Don't get a horn that has a drastic curve in the nose, nor one that is solid a long way back. That just makes drilling the spout a more difficult matter than it needs to be.
A blatant bit of self promotion here, check out our website for your horn.
http://www.powderhornsandmore.com It will give you a good idea of different horns available.
John