Summarize:

Before you begin assembling your gun, you'll need certain supplies. To start, gathering the following:  Several pieces of durable paper (8.5x11", any color) Scotch tape Scissors Ruler Marker Hot Glue A rubber band To start, select one piece of paper. Roll the paper into a thin tube. Gently roll the origami paper into a cylinder shape, leaving a space in the middle if possible. It should look something like a small cardboard paper towel roll. This is your mold, and you'll use it to help roll the rest of the paper tubes needed to make a paper pistol. It should be about the diameter of a pencil. If in trouble, you can even use a pen or pencil to help you get the shape right. To make your gun, you'll be sliding the first tube into a second, larger tube in order to shoot. To create the second tube, roll a second piece of origami paper around the first tube you created. When the second paper is completely rolled up, gently slide the first tube out of your new tube. You will now have another tube, slightly bigger than your first. Like the first tube, this tube should something like a cardboard paper towel roll. Once you've rolled up your tube, secure the tube with your scotch tape. Place one piece of tape on either end of the tube to keep it together. Then, trim down the sides of the tube using your scissors. You want the sides to be smooth and even, with no awkward bits of paper slipping out on the sides. Keep rolling your tubes up the same as the first time. Then, use scissors, a ruler, and a marker to cut your tubes into the following lengths.   For the barrel: You need to have two 15cm tubes.  For the handle: You need to have seven 5cm tubes  For the trigger: You need to have one 8 cm tube. Stack all of the tubes on top of each other, then slightly offset them by moving the bottom ones to the right, diagonally (this mimics the shape of a normal pistol handle). Hot glue them, one on top of the other, together to make a long, thin handle for your gun. You can also glue them on straight, then cut one end of the handle diagonally, forming a slight angle. Trim down the angle with your scissors to smooth out the sides. The excess tube should be on the side the handle slants towards. In other words, if you were holding the gun up to shoot, the extra 3cm would point back at you. This is going to be the "trigger tube" of the gun. This is the barrel of your pistol, so it needs to point away from you, of course. Line up the back of the barrel with roughly the center of your handle, then hot glue it in place. This time, don't worry about there being a space. Roll up some paper (it is nice if you can use a different color) into round tubes just a bit thinner than your first set. These new ones need to be able to slide in and out of the older tubes. To get them thinner, roll them without the guide. You should also cut off the last 4-5" triangular inches of paper as you're rolling the tubes to decrease the thickening paper in the center of the tube. Trim away the excess so that there is only 1/4" of tube poking out of the back of the trigger chamber and nothing poking out of the back of the handle. The bend in the U is on the barrel side. This will be your trigger -- when you pull the trigger back, this little extra bit should pop out the back of the trigger chamber. Make sure that the tube can slide freely back and forth. This is your trigger, after all. Bend the tube into an S-shape, flattening the curvy parts as needed. Slide one end of the paper into the second highest tube of the handle (right below your trigger), letting the curve in the "S" become a little trigger guard. Hot glue the leftover tube to the bottom of the barrel and cut off any excess. This tube should be flattened into a long, thin rectangular strip. You then hot glue it to the back of the handle, curving it around the front and under your trigger guard, if you have one. The goal is to close off all of the unnecessary openings in the tubes on the handle of the gun, but the essential one to close is the one right behind the trigger.   DO NOT close up the trigger chamber. This needs to be open to load and fire the gun. In the end, you'll create an "outline" of sorts of the bottom of the gun. This is why using another color looks so good. Remove the trigger, and put in the spring so that it presses against the outline tube. This will allow your trigger to spring back automatically after you shoot the gun. Fold a piece of origami paper in half twice, forming a long, thin line of paper. Roll this line into a squat tube. Secure the tube with a single piece of scotch tape and trim down the sides to smooth them out. This should look like a tiny toilet paper roll. Then...  Take your scissors and cut the tube open. Then, take your rubber band and place it inside the tube.  Tape the tube back together. You should now have a small, squat paper roll with a rubber band running through it. This is the trigger of your gun. Cut the ends so that, with the rubber banded end as close to the back of the barrel as you can get it, not part of the tube pokes out of the front of barrel. Make sure the back of the trigger rests right at the opening of trigger chamber. When you pull the trigger back, the bit of tube that pokes out unhooks the trigger, which releases the rubber band and shoots your bullets. Now, your gun should shoot. Roll bits of origami paper into small balls. Place a ball into the end of the gun opposite the handle and trigger, and hook the rubber band up. Pull the trigger to release the rubber band, shooting the trigger tube forward and firing your bullets. The ball should shoot out of the gun.
Gather your supplies. Roll your first piece of paper into a cylinder, starting in the corner and working up. Roll a second piece of paper around the first. Adjust and secure the tube with a piece of tape. Roll up two more identical tubes, then cut them to the following lengths. Make a handle by hot gluing all of the 5cm tubes together at a slight angle. Glue the 8cm piece on top of the handle, letting the extra 3cm hang over the right edge. Glue the two long, 15cm tubes together, then attach them to the top of the gun. Roll two thinner paper tubes. Bend a thin tube into a U so that it slides into the trigger tube and the top tube of the handle. Optional -- make a trigger guard with another thin paper tube. Flatten a thin paper tube with your fingers, then use it to "close off" the back of the handle. Take the spring out of an old pen and place it in the top tube of the barrel. Make the rubber band trigger and firing mechanism. Slide your rubber-band trigger into the bottom tube of the barrel. Hook the rubber band up to the front of the barrel, so it is notched between the two tubes. Load and shoot your paper bullets.