Rubber Band Gun - Interchangeable Barrels

Update 6/5/18 I tightened the fit at the leading edge of all four barrels and replaced all four. If you have already printed some that are too loose, use a hair dryer or heat gun to soften the plastic and move the bottom edge a mm or two, to tighten the fit.
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I was looking for a rubber band gun, and the original design of this one and its predecessor were appealing. As I looked at the original and the ones that had been made, a few changes came to mind, and here is the resu