Let me preface this by stating that there are many many ways to do the breech. This is one. If you have never seen one in person, BUY ONE FIRST or be prepared to learn on your first one and fix it. I have made many, many cannons over the years. And to be frank, if your time is worth anything at all, and you want this kind of cannon... order them from BC or others who make them. these cannons are dynamic devices, where the two adjustments that are available interact with the co2 pressure, piston mass, and spring stiffness to function. If any is too far off, the cannon will not function properly. Too stiff a spring and you may see multiple shots per cco2 burst. Get everything completely wrong, and you can full auto (both controlled and uncontrolled). It takes trial and error. First off, you will need tools and materials: Solder flux teflon tape 1/4" thick wall tubing. Found in home depot commonly 3/32" OD brass tubing. hobby shops, mcmaster.com or smallparts.com 1/4" T fitting 1/4" RT Angle fitting Caps Appropriate sized O-rings (depends on breech design) one for the breech, one for the fill port cap. Solid Rivet (see www.mcmaster.com) Rivet shown is a BC rivet Spring (see www.mcmaster.com or www.smallparts.com) the spring shown is a BC spring the brass fittings are either flare fittings or compression fittings, depending on what you have available. The T is always a compression fitting. and two of the handy ones: a RCBS deburring tool and a small tube tubing cutter Take a look at the base of the cannon. The spring should be chosen to fit over the piston and the T fitting sometimes has to be bored out appropriately. It all depends on what is available. This is one method of creating the breech. All that needs to be done is provide a way to compress the o-ring in an adjustable manner. the o-ring should be sized so the bb will just drop through it when it is not compressed. Again, what you make depends on what is available to you. Now this last photo is the main part of the cannon, partly assembled. the cap at the base of the T can be screwed in to adjust where the piston sits and the preload on the spring. If you are making your own for the first time, cut the spring long and then shorten as needed. that is a short anatomy of a fast gun cannon. that particular one is a 5 year old BC cannon. General operation is as follows. gas flows to cap at bottom of T. This pushes piston up, bb sitting on top of it. gas then flows both around the piston and down to end of magazine, pushing BB's forward till they are stopped at the piston. while this is happening, the bb on top of the piston moves up to the o-ring in the breech and stops. Pressure builds up and bb is forced through breech, firing out barrel. common issues I see: Over-tweaking: no bb's fire. or you can multifire as the pressure rises enough in the magazine that the pressure difference between the base of the piston and the top is not sufficient to keep piston up. piston drops and another bb moves into place. First bb fires. process repeats. this error, while cool is difficult to repeatedly produce, although I have done it accidentally. Fit around piston too loose: symptoms can be similar to over tweaking. larger gaps = lower pressure drop across piston = lower max transient pressure inside cannon before piston falls. Way too loose and spring can sometimes jam. Fit around piston too tight: Piston jams in up position, cannon does not fire repeatedly. Cannon fires multiple rounds and then stops: either overtweaking or the piston is too short, allowing two bb's to pass by before moving up enough. Can be a symptom of loose fit on piston. Poor feeding: Not having at least a 10 degree angle WRT the horizontal on last inch of magazine leading into T fitting. Kinked tubing. bad/rusted bb's. lack of cleaning cannons out after day of battling. BB jams in barrel: barrel dirty or dented. bought wrong size of barrel. some barrels will only work with some brands of BB's. copperheads tend to be a skosh smaller, daisy competition have always been a bit bigger when I bought them. Mcmaster thick wall tubing tends to be a bit smaller on the ID than the tube from smallparts. BB's: check and sort BB's. I have seen everything from nicely round BB's to bb's that only approach round when viewed from more than 10 feet away. I hope this helps, please add other advice if it is available.