I'll chime in here as a pipe maker but also as a gunsmith that often works on walnut stocks. I wouldn't suggest using walnut for two reasons. First, it is soft and wouldn't hold up long under repeated firing. Second, it is slightly toxic. It has a chemical called Juglone (you can look this up yourself), and walnut sawdust could cause an allergic skin/breathing reaction to some people. I have never made or smoked a walnut pipe, nor have ever had a skin reaction to walnut dust, that doesn't mean that it wouldn't for someone more sensitive to this specific toxin. You are free to do what you want if you are making this pipe for yourself, but I wouldn't recommend it, especially if you plan to sell it.