A block trigger muzzle-loading K'nex pistol. These are my favorite K'nex guns, both because of ease of use and high power. It even has a sight-ish piece on the side, although I've found it easier just to aim by pointing the gun in the general direction of the target from about chest-height.

By flipping the trigger over to the other side you can make a left-handed version of the gun. You also have to flip the blue and silver rings and the sight.

Modifications and suggestions are welcome. Just make sure to leave a comment!

Note: You need to look at all of the pictures to know exactly what I'm telling you to do. Some steps are rather hard to explain, and are best shown.

Step 1: Gather the Parts

Note: Rods are the long, thin pieces and connectors are the pieces they snap into.
Normal colors will be given first, then metallic colors. I used metallic colors where possible when building this particular gun.

For this Instructable you will need:

13 yellow or grey half-circle connectors
2 orange half-circle with knob connectors
2 purple or silver half-circle notched connectors
3 red or dark speckled grey quarter-circle connectors
3 grey one-opening connectors

1 green or black small rod
3 white or silver small rods
2 yellow or dark grey medium rods
3 red or green long rods (note: orange rods are the same size, but they cause too much friction to be used for the pin and do not fit as the side bars. Use of them is inadvisable.)

3 small blue rings
1 large silver ring

2 fat brown rubber bands (#64)
1 long rubber band

Step 2: Begin Building the Back End of the Barrel

1. Snap five grey half-circle pieces onto one of the green medium-sized rods. (Diagram I)
2. Snap another green rod onto the other side of the grey half-circles. (Diagram II)
3. Snap four more grey half-circles in between the two green rods. (Diagram III)

Step 3: Make the Trigger and Attach It to the Gun

1. Slide one of the blue rings onto the left green rod. (Diagram I)
2. Slide another blue ring and the silver large ring onto the other green rod. (Diagram I)
3. Attach a black small rod and a silver small rod onto opposite sides of one of the orange half-circles. (Diagram II)
Note: the orange piece is essential to the proper functioning of the gun. If you don't have one of these, stop now or figure out an alternative. It is possible to use another grey half-circle and a small rubber band wrapped around the handle. If you find another way, plaese tell me!
4. Slide the trigger onto the left green rod, black rod facing towards the center of the gun and round part facing up. (Diagram III)

Step 4: Attach Barrel

1. Slip another blue ring onto the left green rod in front of the trigger. (Diagram I)
2. Snap three more grey half-circles in between the green rods. (Diagram I)
3. Snap one orange half-circle in between the two green rods. (Diagram I)
Note: You don't need to do step three if you aren't planning on putting more than three brown rubber bands on the gun. If you do put more than three on, the pin bends alarmingly.
4. Add three dark grey quarter-circle connectors in front of the orange connector in alternating fashion. Make sure there are two on the right, one on the left. (Diagram II)
Note: Disregard the light grey connectors. You will need to add them later, but adding them this early was a screw-up on my part. You need to take them off to get the rubber bands on.

Step 5: Make the Pin

1. Attach two silver notched connectors on either side of a grey half-circle with a silver small rod. Make sure both notched connectors are facing the same way. (Diagram II)
Note: If you feel like this grip just doesn't do it, use an orange half-circle with knob instead of the grey half-circle.
2. Attach another silver small rod onto the set of connectors. (Diagram I)
3. Attach a green rod onto the grey half-circle on the non-notched end of the set of connectors. (Diagram II)
4. Insert the pin into the barrel from the back of the gun. The knob of the orange half-circle should go under it, and the small black rod should go over it. (Diagram III)

Step 6: Add Rubber Bands

1. Attach the two dark grey medium rods just behind the trigger, sticking straight down (Diagram I)
2. Wrap the rubber band all around the gun, making sure to put it in the notched bit at the end of the pin and to put it in the long groove going all the way around the gun. (Diagram II)
3. Get the rubber band under the knob on the orange half-circle. If you need to, use a rod you aren't using in the gun to do this. (Diagram II)
Note: Step three is very important. It makes the trigger flip up when you pull the pin back, blocking the pin.
4. Add another rubber band over the first one. You don't need to put this one under the orange knob, luckily.

Step 7: Add the Sight/rubber Band Holders/structural Reinforcers

1. Add those three light grey connectors I talked about in step four. Make the one on the right sticking out, and the two on the left almost flush with the body of the gun. (Diagrams I and II)

Step 8: Add the Last Rubber Band

1. Stretch the long rubber band across the top of the gun, loop it around the notch of the orange half-circle at the front, twist it, and attach it back to the pin. You can actually put it a notch lower on the trigger than I have it here. It'll bend the pin less when you have the gun cocked. (Diagram I)

Step 9: Fire It

1. Hold the gun like Diagram I, making sure not to block the movement of the trigger or the pin.
2. Pull back on the pin until the trigger rotates to the right. You should hear a "click" when it does this.
Note: You may have to pull the pin back pretty hard to get it past the trigger. If you don't get it past on the first try, just try again, pulling faster. (Movie)
3. Load the projectile. You can use almost any K'nex rod, although the smaller ones tend to work better. (Diagram II)
4. Hold the gun like Diagram III and point it at your target.
5. Pull down the trigger with your thumb. You should hear a click as the back of the pin slams into the rest of the gun and feel the gun jerk a little as the projectile flies forward. If not, you've probably done something wrong or the gun is broken. That would be troubleshooting time. (Diagram IV)
6. Watch as your enemy is hit by the projectile.