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To use for fishing, I would just make a pvc one with a BBQ piezo from Bunnings (really cheap). Be much easier to lug over dunes etc

I had a coil and a hot little motorbike sparkplug. I'm trying to remember where it is at the moment, mate was using it up north last I recall

It's not like gunpowder, it doesn't 'create' its own oxygen- google stoicometric ratio. Overcharge will result in less pressure or even no bang.

I made a steel one a while back. Would fire a frozen onion through both sides of a wheelie bin

https://www.ebay.com/itm/9-Liter-BEAD-BAZOOKA-BB9LM-Tommy-Gun-Tire-Inflator-Bead-Seating-Tool-/232487131220

These are great, just a bit expensive. Nz retailer wants $400 odd dollars for them....