Before you begin: A pen spring is surprisingly strong. If you launch LEGO parts into your eyes I will assume that you meant to blind yourself.
Sometimes you find two very different things that work perfectly together. Sometimes those two things are a spring from an ink pen and LEGO. These springs are ideal. A 1/8" dia. rod fits perfectly inside, and the hole in technic parts fit perfectly outside. I could have used the spring for a shock absorber or a little LEGO jack in the box, But a cannon is...A CANNON!
Parts you need:
Pen with spring.
9/64" drill bit. Use a 1/8" drill bit if you don't have 9/64".
1x1 technic brick qty. 6.
black technic pin that you can fit a 1/8" rod through.
gray short technic pin with stud
1/8" rod with stopper qty 2.
1x6 plate qty 2.
parts to make the base.
About this creation
Take apart a pen and get the spring out.
Put it on one of the 1/8" rods. This is the "firing pin" assembly.
The black pins have a bump inside to grab a 1/8" rod. Use the 9/64" drill bit to gently file off the bump without damaging the rest of the pin. You need just enough room for a 1/8" rod to slide through this part easily. When you use the 9/64" bit, that gives the rod 1/128" of play all the way around.
Smooth out the inside of the short gray pin in the same way as the black one.
This picture and the next two show the order of assembly.
This is the exploded view showing all the parts. The fore left is the muzzle end. The back right is the breech end.
Put the black pin into two technic 1x1 bricks. This is the barrel assembly. Put the gray pin in a technic 1x1 brick. The stud end will face the front of the cannon.
Stick a technic brick on the stud end of the short gray pin as shown on the right side of this pic. This is the "breech" assembly.
Put the breech assembly and two 1x1 technic bricks on the 1x6 plate.
Sick the firing pin assembly in through the front. The long end should protrude through the breech.
Push the firing pin backwards. There will be tension on the spring. Use something to hold the pin in. Don't point it at your eyes.
Put the barrel assembly on the front two studs of the 1x6.
Put another 1x6 on top of the assembly and then place the cannon on your choice of mount.
The projectile can be anything with a bit of 1/8" rod sticking out the back. Here is how to make a rocket.
Put the rod in so the stopper stops on something strong, like the body of the rocket.
The other parts are simple enough to put together.
hold back the firing pin on the cannon.
load the projectile from the muzzle end.
release the pin to fire.
The rocket shown can be fired two feet into the air if fired straight up.
This isn't the only way to make a cannon. You can use 1x2 technic bricks for a bulkier, but more substantial cannon. You can use one of the old style antennas as the firing pin. You can make the bore larger to use technic cross axle ammunition. You can make a spring catapault to launch race cars or jets.
I have included a LDD file for you to take apart that shows the basic parts layout.
Quoting Zach Sykes; Master of all Things Mars
Thats genius! I dont like to mess with pieces but its such a simple modification for such a cool cannon. I have to go back to hard-building now just to make something to use that on.
The best part is that the modification is so tiny you would never notice if you were to use the modified parts as they were intended.