Piston Valve Pistol...

[BeaverRat]

Hello,
I have successfully made 3 piston valve cannons now - 1, 1.5 inch porter and 2, 3/4 inch porters. However, they all share a barrel length of 5 feet. I was wondering if it is possible to make a cannon with a barel of say 1 foot achieve similar power?
Thanks

[clemsonguy1125]

Yes, it is, but you will need higher pressure, and an effiecent valve.
Take a look at this

[jackssmirkingrevenge]

cutting down on barrel length will inevitably involve a corresponding loss of power, especially to 20% as you're suggesting so you will need to up pressure, valve efficiency etc. to retain comparable power

[jrrdw]

Something like this maybe...

[hi]

I think we need to think about this a little more. Higher pressure and a more efficient valve is a must, but there may be a point where a longer barrel simply doesn't yield more velocity.

I know that in real firearms, shotguns in particular, after about 12-15'' of barrel length the muzzle velocity remains basically the same. The only reason people use longer barrels is for balance and weight. A shotgun with a 36'' barrel has the same muzzle velocity and is capable of shooting the same pattern as a shotgun with a 15'' barrel.

You would notice a huge difference in velocity from a 1 inch barrel and 1 foot barrel, however i don't think you would see as huge of a difference in velocity between a 4 foot barrel and a 5 foot barrel.

The projectile has a lot to do with it as well. If it fits tightly and have little friction inside the barrel, it will gain speed quickly. If its loose and produces high levels of friction, it wont gain speed quickly.

I think its possible, however i think maybe a 2 foot barrel is more realistic. I guess the only way to find out is to try it.

[jackssmirkingrevenge]

I know that in real firearms, shotguns in particular, after about 12-15'' of barrel length the muzzle velocity remains basically the same. The only reason people use longer barrels is for balance and weight. A shotgun with a 36'' barrel has the same muzzle velocity and is capable of shooting the same pattern as a shotgun with a 15'' barrel.

You would notice a huge difference in velocity from a 1 inch barrel and 1 foot barrel, however i don't think you would see as huge of a difference in velocity between a 4 foot barrel and a 5 foot barrel.

Firearms like spudguns have an optimal barrel length, past which velocity actually starts to decrease. For example, the Colt M1911 fires a standard 45 ACP ball round at around 850 feet per second from a 5 inch barrel. Using the same cartridge for a 10.5 inch barrel, the same cartridge achieves around 920 fps - that's almost 20% more muzzle energy.

Spudguns usually have C:B ratios that will allow the projectile to accelerate if the barrel is extended, there are diminishing returns but lopping off 1 foot off your barrel will certainly have a noticeable effect on power, in some situations more than others.

[BeaverRat]

Ok, I guess I need high pressure and an efficient valve to achieve good results with a short barrel... Obviously a PVC piston valve is out of the question due to pressure constraints and I have no idea how to build a piston valve out of copper or steel.

I plan on having a 3/4 inch barrel, so what about using a commercial 3/4 inch QEV from McMaster? Those can take 300 PSI correct? How does a QEV compare against a homemade copper or steel piston valve of same size porting?

Thanks

[jackssmirkingrevenge]

A QEV essentially is a piston valve, with an extremely lightweight synthetic piston/diaphragm, so unless you're making really brilliant pistons, it will almost certainly be better than a homemade one of similar dimensions.

[BeaverRat]

How come I don't see more QEV Cannos then? I see a lot of small caliber piston valves that could be using QEV's instead.

Can they take 300 PSI?

[jackssmirkingrevenge]

How come I don't see more QEV Cannos then? I see a lot of small caliber piston valves that could be using QEV's instead.

QEVs are usually not that cheap and not so compact either, for example I would certainly not use one in a pen gun - there are plenty of reasons to make your own piston valve instead of using a QEV.

Can they take 300 PSI?

There are people on the forum who have used them to twice that without incident.

[Crna Legija]

Most of the time you have to order them online, also most home made valves have more flow then a qev because they have supports in the barrel port but lack the the light and one way flow piston that qevs have.
and some people just like to make stuff and op for a home made valve.

I think someone took a qev to 700psi. the piston would burst before the shell anyway, i still don't reconmend it.