Why Knex bullpups are (mostly) useless and a stupid idea. Answered
So in the real world the point of a bullpup design is to either reduce the overall length of a gun, or to have the same length of gun but with a longer barrel, which results in a higher velocity bullet which can go further. Another often added bonus is that the gun is more balanced weight wise around the handle, making it easier to handle.
Now to crush your dreams.
Not a single on of those advantages carries over to knex guns. Not one. A bullpup knex gun will only end up longer(yes LONGER), use more parts, have theoretically lower range, it is less balanced, and more complicated with more things that can go wrong.
With a knex gun, you want as short of a barrel as possible for the shot to go through, so that there is as little friction as possible on it. This is one reason that turret guns can be so powerful. Now with a bullpup design, you almost automatically have to add a longer barrel, as the shot has to leave the action area AND go over your handle/trigger mech. Not good.
The gun will be less balanced because now basically the entire gun is behind the handle, unless you put some fat barrel on the front to balance it out, but then that breaks the first point I talked about, as well as using many more parts and defeating the purpose of being a shorter weapon. Most ram guns now are mostly balanced, as the handle is usually in the middle of the gun, as the ramrod has to be pulled behind it as well as going in front of it, so you place the handle and trigger as close to the middle so as to make the trigger simpler.
Now as to why making a knex bullpup will make it longer, just look at the pictures. It will make sense then. Also this is why you need many more parts for a gun like this. Also the stock becomes disgustingly long.
And finally, this is more of a design issue than a complete flaw, but the trigger is usually(USUALLY) weaker and more prone to failure/not working correctly with a bullpup gun.
Now this doesn't apply to everthing. Slingshot guns and repeating slingshot guns actually benefit from a bullpup design, that will actually make them shorter and more balanced, because they are more similar to real guns in their layout in that the shot/magazine is the farthest part back in the mechanism. Also replicas, well duh.
The pictures should make this all make more sense. And I assumed a build for performance, so the pin draw is long and the magazine is as far away from the pin as possible. Because all knex guns should be built like that.