It fires via weighted impact (typical) and features recoil dampening and minimal-friction loading systems not seen elsewhere. My entrant to any applicable K'nex gun contests.
A few exotic parts were used, firstly a metal spring that fits around a K'nex rod. Also, to make this, you will need to mutilate three dark grey connectors by cutting off their tops (remove half of the circle). Though you could get the spring at any hardware store, you can use rubber bands in its place.
Step 1: -back End- the Pile Driving Shaft.
On the SPRING - it is extremely effective to use one here. You can get springs like these that wrap around K'nex rods at hardware stores (best bet) or from hobby stores (remote control cars) This is how the power is converted.
GREY ROD + ORANGE CONNECTOR -> BLUE ROD -> RED ROD (Projectile)
(Several rubber bands) (spring-loaded)
Let's save adding the grey rod for last.
Because the blue and red rods together still have less total mass than the grey rod hitting them, their velocity will match whatever speed you can get the grey rod to reach, assuming the laws of physics and transferrence of energy. The spring acts as a shock absorber, preventing any extreme stresses on the K'nex parts and returning to optimal position again.
Yours may need to be tuned differently from mine to keep the blue rod in the perfect place, but you'll understand this better once it's complete.
If you don't want to use a spring, rubber bands (fourth picture) are an alternative, although I've so far not had much luck with them. Loss of strengh.
Step 2: -middle Section- the Handle.
Step 3: Attach -back End- to -middle Section- (Like So)
Once again, follow the pictures. It's looking vaguely like something, now.
Step 4: -front End- the Ammunition Hopper, Loading Arms, and Barrel.
Either way, I've split it into the hopper lid (top and first image) and the barrel with the loading arms. Try not to get your front/back mixed up, take your time.
This section is complicated, so I'm trying my best to explain it through pictures.
This is where you use them -- the three decapitated dark grey connectors should line the bottom interior.
EDIT: I have added (as the final picture) A picture showing how I create the decapitated grey connectors necessary for the hopper. I place a row of greys on a rod and then clamp it down with a vice. Then, carefully, I cut downward. Any saw meant for wood should be able to cut through K'nex.
Step 5: Attach -middle Section- to -front End- (Like So)
Step 6: Structural Reinforcing and Arming.
If you've made it this far, horay! Construction complete.
Now to figure out how exactly to use it.
Step 7: Firing Procedure.
Before every shot, you must pull the loading arms back to allow the stack of red rods to drop down, putting one in a ready-to-fire position.
Then, by pushing forward on the loading arms, it should tuck the black pieces between the ready-to-fire round and the others, lifting the rest up and away. At this point, there should be so little resistance that the red rod could slide out if the gun were tilted down.
Pull back, let go. So long as the blue rod is kept from blocking the falling of the red rods, the gun has an easy time supporting powerful rubber bands. The only limitation is that red rods lose their aerodynamics rather quickly once they begin to fly through the air sideways. At close range, however, they have serious inertia and I warn everyone that it will dent walls.
If you want even more serious power and better range, fill the hopper with TAN rods if you have enough and let the grey rod you pull back be replaced by a heavier black. Just don't hurt yourself or the cat.