This has been posted here before. Here is the explanation given by the video poster:
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civilwargeneral
10 months ago
Right, I had a bunch of old powder and so used some PVC pipe and scrap lumber to make the cannon and get rid of the powder. Wouldn't dare do that with a real one, way to expensive among other things. Also, we set the camera's up and started them filming before we even lit the fuse, then got far away.