A cool idea,and I have to say angle seat valve is excellent.
I have made sth similar https://www.spudfiles.com/viewtopic.ph ... 49#p387349

Guess you will need to add some fin to steady the shell,and a sabot,like APFSDS shot fot tank gun and it will have a good penetration ability.

Perhaps pneumatic direcional valve for IG541 fire suppression system can do it,as it can handle 2,500psi at 3' or 750 psi at 6'

Note that it could be difficult and dangerous to make such a stuff.If you just want something can shoot,built an air gun is a better idea for you,as it is easy to make and operate.

I've built this one a few months ago .Originally it was designed as a ropethrower for rescue purpose,etc but it can also launch many other shell. Basic data weigh appr.6kg/14lb length 1.3meter/51in chamber volume 1.45L/88in3 muzzle speed /at 1.2Mpa/175psi,CO2 ropethrowing shell 50mm/250gram 55m/s or...

Well,exactly they are much cheaper in China. https://item.taobao.com/item.htm?id=564983254559&ali_refid=a3_430585_1006:1106453081:N:%E9%AB%98%E5%8E%8B%E6%B0%94%E6%B3%B540mpa+%E7%94%B5%E5%8A%A8:cd5a9a0d37f2f659b2068fc87f821f3c&ali_trackid=1_cd5a9a0d37f2f659b2068fc87f821f3c&spm=a230r.1.14.3#detail *Th...

Thank for your guys supporting me.Here are some more photos,hope them can help you if you wang to try it

You may use the 4*6 PTFE tube,or gel ink refill and make them up by tape.
I've used the gel ink refill as barrel in my program
http://www.spudfiles.com/miniature-cann ... ml#p383803

Recently I've found that a syring [or a injector] could fit well with the 6*4mm[.25*.15in]plastic tube and it could use as a mini cannon.It is easy to bulit and could be use for a lot of times.The one I'm using now has operated for about 1,000 times ,and it's still operationable.The metal barrel was...