Back in the day, I thought Trigun was pretty cool -- part western, part comedy, all anime. It's more or less a fond memory these days, but when I saw the gun and saw that it had moving parts I thought it would be cool to build; and I was half right. The template looks like a geometry test and the lack of textures makes it feel kind of bland, but the fact that the chamber and bullets are removable and the gun actually has a hinge to open it makes it probably the most interesting papercraft I have built to date.
I got this through a popular Korean papercraft site that has long since shuttered its doors to foreigners. It's a shame because a lot of good hand designed models come from there.
Probably the biggest challenge was building this thing straight. The design strikes me as a scratch build that was later dismantled and unfolded into a template. The parts glue on top of each other and making sure it wasn't (more) crooked required me to build it almost twice. The mechanism for the hinge is not very strong, either; I recall having to reinforce it somehow. All of the "extra" paper -- meaning the portions that could have been one part and were instead multiple parts glued together -- places way too much weight on the front of the gun and causes it to sag when the barrel lock is not in place.
As an aside, whenever I show anyone the gun in pictures they always ask me the same asinine question: "Does it shoot paper bullets too? Hurr hurr". Then I show them the paper bullets that it does not shoot and the fact that it moves -- and they look at me like I'm Yomiko Readman and I just used paper to block bullets and cut steel.