SpudFiles

here is my idea to build a semi auto potato cannon. it uses a gas piston to hit a valve to release propane into the combustion chamber and to load the next round into the firing chamber. it is limited to semi auto because the plans include a grill ignighter if you used a tazor you would need to have a cut off switch that wont click on until the bolt is closed back up







in first figure is the cannon ready to fire. all primed up.

in second figure the cannon fires and the pressure inside the barrel pushes the piston letting the next ball enter. and not included in the sketch the propane valve opening.

in third figure the cannon is ready for next shot. please note that this is just an idea I still haven't confirmed what the material will be. but I am thinking about using copper because after a few shots pvc would hot and possibly fail. also copper would work better for all the moving parts. and also there will be a cocking handle on the piston rod so that it can be cocked for first shot

It looks good on paper, but the problem is, not enough pressure would be generated to move said piston enough. This has been discussed numerous times here, but maybe you can get it to work. Good luck.

the pressure thing occured to me too i was thinking of having the piston closer to the chamber then specified. and also it would have a large combustion chamber. that way there would be much more pressure.

So just how are you planning on letting more air in?

A larger chamber is not necessarily going to generate more pressure. There will be more volume, but not more pressure.

every spudder have thought about achieving semi or auto in this way... I am suprised that JSR haven't posted a response yet

since he hasn't done that.... well the main problem here is that firearms based on this principle of operation use much higher pressures (in the order of tens of thousands of psi)
trust me.. it's not likely to work well

not to mention, their ammo has the fuel and the oxidizer in the casing, so you don't need to deal with all of those complications.

has anyone actually tryed to build a cannon like this ? or is this still un tested

nibbler125 wrote:has anyone actually tryed to build a cannon like this ? or is this still un tested

Well four responses above your post say it wont work as is. So my educated guess would be no, no one has built a combustion spudgun according to your design.

you would be better angling the gas tube foreward, and having a foreward moving piston so the gas keeps its velocity as it moves up to the piston

POLAND_SPUD wrote:every spudder have thought about achieving semi or auto in this way... I am suprised that JSR haven't posted a response yet

Well sorry for being so predictable and for being away in Madrid for the weekend hehe

As above, you shouldn't be looking at gas systems for inspiration as these rely on pressures over a thousand times greater than those a combustion spudgun generates in order to function. Rather, have a look at direct blowback.

My attempts at making a semi combustion centred around the latter system but using preloaded cartridges, though I'm sure you could adapt the design for propane/air injection. Another idea would be to use the blow-forward boltbut substituting the air supply for a combustion chamber.

There's very little point to this.... fueling the combustion would take a lot longer than just muzzle loading.