A very easy to build K'NEX gun using only basic pieces and rubberbands that is surprisingly strong and extremely powerful. This may be a toy but it is much more powerful than your average dart gun so be careful with it.

Build time: ~10-15min
Distance: estimated at 70-80ft (this has not been tested nor have I put the maximum amount of rubberbands on the gun)

Pieces needed:
Connectors:

11 white
7 yellow
6-7 green
8 reds
1 light gray
2 dark gray
1 orange

Rods

2 blue
19 white
34 green

Firing Pin

1 black rod (length of gray rod but stronger)
1 white rod
1 orange connector
1 tan clip

Non-K'NEX parts

black tape (aka electrical insulation tape)
rubberbands (I got mine from the Hobby Lobby, they're made to hold the wing on a model plane, but I'm sure the broccoli bands would work)

Step 1: Barrel

These are the parts for the barrel, first build the three layers then attach together at every indicated spot. It may be easier to just build the center portion then attach each white piece one by one.

One additional thing, there is a green piece connected in the center at the spot marked (0), this piece is not require but it really helps to keep the firing pin from jumping out of the track when it is cocked. I also wrapped this one rod in black tape to help keep it in nice and tight.

Step 2: Firing Mechanism

I've provided multiple pictures so it can be more easily constructed. Just connect the catch to the trigger with a white connector and hinge it all on a second white connector. Also note that I wrapped the trigger in black tape; this is highly suggested to reinforce the trigger.

Don't forget the rubberband to hold the firing mechanism down until the trigger is pulled.

Step 3: Grip & Finishing Touches

First, clip four (4) white rods into one (1) white piece as shown in the first photo. Then attach pieces to each side of the white connector as shown so you have a three piece wide grip.

OK, now slide the grip onto the bottom of the barrel as shown in the second photo and secure with a green rod on each side. Then slide a white rod though where indicated; you will have to flex the grip backwards a bit and the rod won't clip in but the flexed grip actually holds the rod in place nicely.

Now, just wrap your rubberbands around the yellow clip (which one depends on your rubberbands) and put in the blue rod at the back to help secure the firing pin.

How did I make my firing pin? with a black rod (same as gray rod but stronger) with a tan clip on it, a white rod with a orange clip, and plenty of black tape...there are much better firing pins out there so I recommend you find one (just search "K'NEX Gun") rather than use mine...

Well, as soon as you have the firing pin in with the rubberbands wrapped around it, just drop a yellow rod into the top of the barrel, cock it, aim and fire away.

ojochris has built a magazine with this gun that doesn't jam https://www.instructables.com/id/Knex-Magazine/?ALLSTEPS

I may add some additional add-ons, such as a proper firing rod and a fieldlight (in the works) if I am so inclined.

Looks kinda like a Beretta, eh?