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I built this last week. It works like a champ and will put a hole through 1/2" thick plywood. Goes several hundred feet in the air. Here's a video:
For all those looking for a tutorial, I'm assuming the OP used this too.
I didn't follow any tutorial directly, I designed my own by studying others. This is a chamber under barrel design, but it's not per any specific plans, I just built to what I wanted to. That is a good starting tutorial though,