See how to make blowdarts out of some cotton and wire. These are actually a bit better for larger diameter blowguns, like 3/4" ID blowpipes, so get your PVC or EMC out and get piping.
You can expect to easily penetrate thin metal, such as spray bottles, pop cans, and even the rain gutters on the side of your house.
These darts aren't the prettiest things, but they do their job well, and they won't fall apart on you, so the only thing you have to worry about is not losing them.
They unfortunately aren't the quickest darts to make either, so be prepared for a time consuming blowdart experience.
Get a piece of particle board and paint a target on it for maximum blowdart fun!
A blowdart is a deadly weapon, so don't shoot where you can't see, don't shoot up, and don't shoot when there are people or animals downrange. Basically, treat it the same as you would treat a firearm or any other weapon. Then, enjoy!