Introducing the Basic Integrated Cannon (BIC). This is a printed cannon, needing only a few pieces of hardware and a standard barrel to complete.
Notice of limitation of liability. This design is provided as is, with no guarantee that it will function as stated or that it will be safe to operate. If you build one of these, you agree that you are liable for all damages that occur due its use, even if those damages are a result of faulty design. If you don’t know what you are doing, don’t try to build one of these.
The BIC integrates a 50 round vertical magazine, breach with provision for an interrupter piston, and up tube all in one printed piece. The prototype has been in two battles, about 16 sorties, and behaves just like a standard fast gun cannon. Bench testing with a hard tweak resulted in BB speeds of up to 265 FPS, so it’s no light weight either. The chronometer also recorded 6+ rounds per second so it should be just as fast as most fast gun cannon designs.
From the picture of the interior below, you can see that the magazine portion is a vertical ziz-zag pattern, with built in slopes to help the BB roll to the bottom. The slope to the back is 20 degrees, while the slope to the front is 10 degrees. This means that even when the gun is tilted down 15 degrees, there is still a minimum 5 degree slope to keep the BBs moving down. For tilts more than 15 degrees, bench testing shows that the pressure feed at the top of the magazine will reliably move the BBs into the breach area. A magnet can be glued on the outside to help move the BBs to the breach if needed.
Magazine pressure line
The CO2 connection from the piston area to top of the magazine is internal, so there is no need to mess with a separate hose.
A “T” is molded in at the bottom of the magazine. A standard fast gun piston interrupter and spring are used. A 1/8” NPT thread is used to seal the bottom of the “T” and attach a 10-32 nipple for the CO2 line.
Fast gun piston and spring. I used the set available from Strike Models
A 1/8” NPT to 10-32 fitting, Clippard part 15036-pkg shown
1/4-20 nylon bolt
o-ring with 1/4” ID, 1/16” thickness (x-010 size)
Threaded fitting corresponding to the barrel nut you use
About 1” of 1/4”OD x 3/16” ID metal tubing
Loctite 680 retaining compound
Rubber toughened CA glue
1/4” and 3/16” drill bit or reamer (reamer preferred)
1/8” NPT tap
1/8” NPT bottoming tap (can be made from a fitting)
Building the BIC
Print the BIC in ABS with 4 perimeters, 50%+ infill, 3 bottom layers, 6 top layers, and 0.25mm layers. These settings are assuming that you have a 0.4mm nozzle. Adjust the print settings if you are using a different nozzle.
Once printed, you need to treat it inside and out with acetone. I used a cheap airbrush to get it everywhere; you can also pour a little down the magazine and blow it out with air. At this point test to make sure a BB will roll freely down the magazine and pop out the bottom. If it gets stuck in the middle you can apply air to the magazine and hopefully pop loose the obstruction. If that doesn’t work, discard the magazine, adjust your print settings, and try again.
The top of the magazine needs to be threaded ¼-20 for the magazine cap. The recessed part also has to be smoothed over. Glue a piece of sandpaper to the “BIC sander” part and use a drill to smooth it down. Hit it with acetone again and it should be smooth. Cut down the nylon bolt and make sure it bottoms out on the shoulder of the bolt. Add the o-ring to the bolt.
Next are the threads on the bottom of the BIC. These are 1/8” pipe thread and they need to be cleaned up and cut a little deeper. You can start with a regular pipe tap and then follow it up with a 1/8” pipe fitting that has some slots Dremeled in it. The regular pipe tap tends to leave some loose plastic in the area where the piston will slide. This area needs to be scraped smooth so there is nothing for the piston to hang up on. The tap for the magazine pressure line is also in this area, so make sure it is unobstructed.
Finally you need to drill the up tube with a 3/16 drill and drill the top of the gun 1/4" for the barrel tube. Be careful since the plastic will self feed and grab the drill bit if you are not careful. I use a reamer to avoid the grabbing. The depth of the 1/4” hole at the top is 0.55”. If you are not using a reamer, you can put a collar on the drill at this length to prevent it from digging in any deeper. At this point, a BB should roll freely from the breech area through the top of the up tube.
Take the fitting for the barrel nut and drill out the inside to 1/4”. Then glue in the piece of 1/4” metal tubing with the Loctite retaining compound. The tubing should stick out the back of the fitting 5/8”. I use a lathe to cut off the unwanted half of the fitting, but you can use a hacksaw and belt sander to accomplish the task. Using the toughened CA glue, glue the tube into the top of the BIC.
Fit the piston to the breech. This is the tricky part, since you have to do this completely blind. My piston ended up being 9.5mm from the top of the piston to the bottom.
Start with something a little bigger than this and cut it down slowly until it works. To fit the piston, first install the piston and spring followed by the 1/8” NPT fitting. Screw the fitting down. Using an Allen wrench through the hole in the fitting, verify the piston moves up and down freely. With the piston in the down position, see if a BB inserted at the top of the magazine will roll all the way down and then through the up tube. If it is blocked by the retracted piston, remove the piston and shorten it. Reinstall the piston and repeat until the BB roles all the way through the gun. Now you can hold up the piston with the Allen wrench and verify that it blocks the magazine when extended. If everything works, put a few drops of acetone around the fitting to seal it.
Hook up a pressure line and install a barrel to test the gun.
Alternative build configurations
If you don’t like the nylon bolt for the magazine cap, you can drill the top out to 1/4” and glue in a standard magazine fitting and cap.
If you don’t want to use a fitting on the bottom of the gun, you should be able to use a 1/16” ID brass tube as the feed connection. Drill through the provided hole in the front of the magazine and glue in the 1/16” tubing. Replace the 1/8” to 10-32 fitting with a cap. The resulting cannon will be slightly shorter. Caution, this is untested.
License. This design is provided with absolutely no limitations. I can’t be bothered to track you down and hire a lawyer to enforce any license restrictions, so why bother putting any limitations in place? You are welcome to sell it commercially – after all, you will only sell it to my fellow combat friends and that will help the hobby. Courteous people will attribute the design to me, and those that claim the design as their own aren’t very nice I don’t want to be their friend anyway.