This is a simple, easy-to-build true trigger pistol made of K'nex. Its range is okay (15 feet), but it is light and requires few pieces.

Step 1: Handle

Build the handle. You need these parts (Colors of the regular, more colorful parts are in parentheses)

8 white (yellow) 5-clip connectors
2 speckled (white?) 2-clip connectors
2 green (green) 4-clip connectors
2 blue (blue) 7-clip connectors
12 black (green) rods
12 silver (white) rods
10 blue (blue) spacers

Build the object in the first picture twice. Put 2 blue spacers each on to 5 silver (white) rods and connect. Those are 1-5 on the second and third pictures.

Step 2: Barrel

Build the barrel. I have to give credit to bakenbitz, I got the general idea from the BBKWG. You need:

10 black (white) 8-clip connectors
6 brown (orange) long 2-clip connectors
4 gray (gray) 1-clip connectors
1 dark gray (red) 3-clip connectors
8 black (green) rods
11 silver (white) rods

Connect 5 black (white) connectors with 4 black (green) rods. Do this twice. Build the parts in picture 2, and connect them with silver (white) rods as in the other pictures.

Step 3: Trigger and Firing Pin

Build the firing pin and trigger. You need:

1 brown (orange) long 2-clip connector
1 blue (blue) small 1-clip connector
1 dark gray (red) 3-clip connector
2 black (black) Y-connectors
1 black/blue hinge connector
1 black (green) rod
3 silver (white) rods
1 blue (blue) rod
1 black (gray) rod
2 blue (blue) spacers
tape

Put the brown (orange) connector on the black (gray) rod, then clip the small blue connector vertically in front of it (see picture). Tape together. The trigger should be easy to build from the pictures.

Step 4: Assembly

Put the gun together. You will need:

Handle
Barrel
Trigger
Trigger holder
Firing pin
3 gray (gray) 1-clip connectors
7 black (green) rods
2 silver (white) rods

Use 4 black (green) rods and 2 silver (white) rods to connect the handle and barrel (see pic. 2 for a close-up). Attach the trigger by running the blue rod with one Y-connector and one blue spacer through the white (yellow) connectors at the top-front of the handle and the 3-clip connector on the trigger. Put the silver (white) rod that is directly connected to the hinge into the gap on the underside of the barrel.
Build the parts in pic. 6, and attach the the barrel as in pic. 7. Push the firing pin into the back.

Step 5: Rubber Bands

Add the rubber bands. You need:

3 medium-sized rubber bands, like the kind you get off of newspapers

or

1 large band and 1 medium band

If you use all medium bands, tie 2 together. Put the knot of this band or the middle of the large band in the brown (orange) clip, and attach the ends to any or the rods on the barrel (the third works for me). The other band holds the trigger up.

Step 6: Sights

Add sights (optional). These can be used on many types on K'nex guns. You need:

1 gray 1-clip connector
1 black male joint connector (it is the same height of the gray connector, and has a ball on the end)
6 Y-connectors
2 silver (white) rods

Connect the gray and ball joint connectors to 1 Y-clip each, and connect them to silver (white) rods as in pic. 1. Attach the other Y-connectors to the barrel (see pic. 2), and connect the parts with the rods to those (see pic. 3).

Step 7: Using the Gun

Pull back the firing pin until the trigger goes up. Insert the ammo into the front (silver/white or blue rods work) and pull the trigger to fire an awesome 15 feet! Note that, as usual, I cannot be responsible for any damage caused by misuse of this gun, short-ranged as it is.