The Liberty One-Shot Pen might also make a great project.
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The 3/4” pipe and shotgun shell project doesn’t require a $5,000 3D printer.... can be created by anyone for about $2. I would like to see a single shot .22 or .25 pen though ,, stealth is VALUABLE.
I would love to have a what appears to be a cell phone but has a working firing mechanics for at least a single shot .22lr.
Could be easy enough making a snap on printed extended battery pack with the guts in it, I have one for my Samsung Galaxy 2.