We are using a series of books written by John Austin as a basic guide: Mini Weapons on Mass Destruction...yes only with a group of homeschoolers would such a book seem like a good idea!
We made blowguns out a pvc pipe and a reducer (to keep the younger kids from sucking the projectile back into their mouths and to produce a tighter seal around the gun). The design is simple, cut a piece of pvc pipe about 12 inches long, attach a reducer (we needed the aid of a hammer to attach this), insert your projectile (we used marshmallows, nerf darts and paintballs) and blow! This is a fairly cheap project as you can get a long piece of pvc pipe for under $3 (I think it was either 8 or 10 feet) and reducers cost about $.30 each. We did find that rolling a piece of paper or two and putting it into the barrel to reduce the diameter helped some of the projectiles fire more efficiently.