Regular plastic milk jugs can be formed with a metal plumb bob or similiar shaped solid metal object into cones for excellent darts, when cut to fit they seal all the air behind them. Using a heat gun or propane torch heat a section of the flat side of the jug till it turns clear and press the plumb bob into the plastic, with a little practice you can turn out neat consistent cones. Mark the cone using the blow tube as a guide or use a sharpened section of the same size tube to cut them against the plum bob or other metal cone shape. Trim if necessary till they just fit. Points can be nails, wire, or bamboo skewers, held in place with epoxy, put a dab inside the cone and outside to prevent the cone slipping on impact. I had good luck with small but heavy ring nails, but can't remember the exact size.
Something else if you leave the lid on the jug and make a lot of cones without removing any or compromising the jug the air inside can help press the plastic onto the plumb bob. I usually didn't but it shoulf make the jug give up more cones.
One of the most powerful blowguns I ever made was a five or six foot emt tube and I used plastic cones and ring nails for darts it had good range and lots of power.
I made lots of paper tubes using paper cones and pins during the long winters back in my high school days. Amazing how accurate these can be.
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