The overall goal was for it to be 'semi' semi automatic, aka didn't need to be refilled between every shot but loading was still done manually. This was going to be achieved with a sprung piston, controlling airflow exiting pilot valve and minimizing airflow from a reservoir to the guns chamber. So the piston would close before pressure could build enough to keep it forced open. Perhaps a cross section could help explain this:
- Section view
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- I had a broken arrow lying around which is the perfect diameter for 6mm bb pellets
- I didn't have money for anything larger
So as to serve as a trial run before building larger and more possibly more complex creations when funding allows, I made this.
The design was inspired by one of wyz2285s guns, found here
Where I live, galvanized fittings are cheaper than PVC (plus as far as I am concerned metal is better than plastic,) so that is why metal was used for a low cost build. Other cost saving measures involve using cheese wax or electrical tape for sealing the threads (I find cheese wax works very well).
The piston is a small section of television antenna or similar aluminium tube with epoxy filling about 1/3 of it. Initially a piece of gumboot was used for the sealing face, but this wore out after awhile. I am currently using some tyre inner tube, which is holding out very well.
The piston (shown below) is lightly sprung which stops leaks forming when initially filling the chamber with air (it still leaks below 20 psi). The piston slides inside the casing of an old AA battery, this along with some electrical tape and oil makes for a near air tight seal.
The screw was an experiment to stop unnecessary air passing the piston rather than exiting the barrel, when piston is fully depressed the intent was for the screws body to fill the main exit hole in the back of the battery. It doesn't work well.
- Piston assembly
Similarly I also have a larger aluminum tube, initially to protect the carbon arrow. As it is overly long, it has a silencing effect when firing. Again I prefer not to use it as the added length makes it too unwieldy and I cant be bothered to cut it down.
The stock is made from 30mm diameter steel tubing. On my quest to save money and not having anything like JB weld on hand, I let a friend who had just got a welder have some practice at welding thin metal. It doesn't bother me, its better than what I could do for the price.
Holes were then cut and filed until the gun was able to slot in place, its an interference fit with the galvanized elbow, and a 'securing' screw holds the hook on the blowgun firm. Its a surprisingly secure fit, yet removal is simple.
- securing screw
Filling is done with a fridge compressor, it was the cheapest option for me, just had to jump into a few skips to salvage some stuff
- compressor
Throughout the build and tuning process many items have been fired and with different methods. The most effective I have found to date is a 4.5mm steel bb with a 6mm lightweight bb as a sabot. This has been measured at 700fps when charged with 130psi, I feel sometimes it fires faster than this however.
The lightweight bb by itself has been recorded at just over 1000 fps with maybe 150psi (it was past the end of the pressure gauge).
Other random damage pictures:
) that JSR broke the sound barrier with a 6mm burst disc pneumatic.
