i've used this cone for years in the field. you have to use a #4 filter (smaller will blow out). you also have to be sure to fold the filter per instructions and also pour the boiling water slowly and evenly into the filter; too fast and you will blow it out. it is still much better than the plastic "oragami" filters from rei...

I got this and tried it at home first, using a #4 filter over a mug. It immediately poured right through the paper, bringing the coffee grounds with it. I was sad... really wanted this to be my camping-coffee solution!