Forgive me that I can't provide more information for something I often take credit for having helped start. For those following the eventually-to-be-fleshed-out guide series, this topic might be a bit advanced for you but I'll try to give a simple run down of the concept. Then we'll be going over my Barackuda in the next post.
So in real firearms, there are different "actions" for making the firearm ready to fire again. I won't get detailed, but one of the manual actions for repeaters is bolt action, where you operate a handle connected to the bolt to cycle the action. Typically this involves pushing a round out of the magazine up the feed ramp into the chamber.
This term was borrowed for K'nex launchers where originally it just meant a basic striker action. Some people would add handles to their strikers to make them easier to charge, some that you would reset back into position like a bolt handle while leaving the striker charged (and this is still a common practice to make strikers easier to charge while not adding weight to them).
However, one certain launcher caught my eye one day. I wish I could remember exactly what it was or by who. It was a bolt action that fed from a magazine above the barrel. There was a pin connected to the handle that pushed a round out of the magazine and into the chamber. This is where things get hazy but I believe it was done more for the novelty of it as the striker's full length wasn't used efficiently. So it gave me an idea.
A problem with simple repeaters is that the rounds, especially rods, will cause a lot of friction and reduce the max range you can get. So I had the idea (though I probably wasn't the first, just as far as I can remember) to utilize a bolt action to dramatically increase the range of repeaters. I wanted to more efficiently utilize the striker length and to have a more traditional bottom-feed magazine. Truth be told, I felt too inexperienced at the time to come up with something alone, so I collaborated with Sol3kill3r to make something. Unfortunately I don't think he ever got pictures, only a video. I'll see if I can track it down somehow, but I believe he unlisted them. Anyway, it was a good start and I was excited to work on the design. However, I believe the story goes that he tipped off builder Zak on what we were working on.
Zak then designed the ZKAR (Zak's K'nex Assault Rifle?) and beat us to the punch. The original post and pictures are since long gone, so I can't show his original design. I was peeved because he made a pretty excellent launcher that was the first bottom-loading bolt action with instructions posted, beating me despite it originally being my idea. His design exceeded what I was capable of at the time, but also had flaws. He refined the design over time and made the ZKAR V2. Luckily, someone else got instructions for it.
While it still wasn't a flawless design, it had set a high bar and was the standard for a long time. I kept trying to design my own bolt action launcher but could never meet my own expectations let alone make something to compete with the ZKAR. I was dead set on making something notably better or novel. One example:
It was a really rough design and just didn't satisfy me, so I never displayed it except as proof that I had designed bolt action myself. It was an attempt to use a removable magazine, but the method I used wasn't good at all. I scrapped this design pretty quickly.
Another design went for the novelty approach:
It was an attempt to design a bullpup (magazine behind the handle and trigger) bolt action, but I didn't have the pieces to finish, and the resulting stock would've probably been too long. I did test fire it and it didn't perform all that great since there was an excessive amount of barrel after the chamber.
And so I went on sulking as a builder who never even made a good example of the idea I originally advocated for. Until just a few years ago, but we'll save that for the next overly long post.