My first spud gun

[jordzan]

Gday this is my first crack at making a gun, only just found this site a few days ago and rushed into making one so have alot of ideas to change/try etc. Would not of been able to make it without Technician1002's help, thanks very much mate.





Also tried a different breech style that could be interesting to someone

It's a 25-15mm reducer from the sprinkler valve to barrel. still need to cut the loading hole etc in the barrel and mount the o-rings.
then some 25mm pipe fit pretty close on the reducer, cut out a shape for the bolt to follow.
25mm to 20 reducer with a bolt through it onto 20mm pipe and a 20-15mm reducer on the end.







[/img]

[qwerty]

Looks a bit.... abstract but other than that it seems like a very nice gun you should paint it to hide all the...er...abstractness

The breech load system looks nice but the barrel in the first picture is a bit short.

Are you just shooting potatos or other stuff. Get a 3/4 marble barrel and you will be impressed with the results.

qwerty

[jackssmirkingrevenge]

Not bad for a first effort, if you're interested in dramatically increased power tap the blowgun directly into the sprinkler valve and use a much longer barrel, you can afford one at least 4 times as long for that chamber volume.

[jordzan]

i know i know it looks horrid, coloured primer+cement / not cleaning anything up = abstract. the barrel that's actually on the gun is just a straight 20mm to use while i made a breech system. the 15mm barrel on the breech system is nearing twice as long.

i plan to shoot anything that looks fun and fits in the barrel, potatoe, batteries, marbels, darts and was thinking about chopping some dowel down into little bullets. 3/4 marbel ... bugger that should cause some damage haha
will have to make a trip to toys r us mwahah

[FORE!!!!]

nice cannon u got there ,i bet she dont leak...

[Mitchza89]

Great job mate, you did very well. I look forward to more guns from you!

Cheers,

Mitch.

[Willdebeers]

If you doubled the length of the barrel you would almost certainly double performance.

[inonickname]

If you doubled the length of the barrel you would almost certainly double performance.

At least, as well as more than doubling the accuracy.

Asides, very nice gun, some paint would complement it nicely! We look forward to more

[jordzan]

Thanks for the comments guys, will definatlly get some paint on it. And many more guns too come. Already started a brass gun

Can i ask how in general the chamber volume vs the barrel length works ? have you guys made that many guns that you just know by now that it needs a longer barrel or is there maths(ewww) and what not ?

[jackssmirkingrevenge]

Can I ask how in general the chamber volume vs the barrel length works ? have you guys made that many guns that you just know by now that it needs a longer barrel or is there maths(ewww) and what not ?

I share your views on maths, however if you're a bit patient you'll find out that in this case, a bit of calculations saves you a lot of effort and allows you to take ful ladvantage of the potential of your launcher.

Basically, while the pressure in the barrel is greater than atmospheric pressure+projectile friction, the projectile will continue to accelerate.

Imagine your chamber and barrel have the same volume, and you're using 150 psi (10 x atmospheric pressure which is approximately 15 psi). This means that (simplifying the calculation) even if your barrel is 10 times the volume of your chamber, your projectile is still going to accelerate and give you more power.

A bigger chamber will alwyas give you more power, but it comes to a point where the gain in power is insignificant compared to the increase in chamber size. Take a look at the virtual launcher I modelled here in GGDT (which by the way is a very useful tool to download when it comes to helping you determine optimum chamber:barrel ratio, get it here)

Starting with a 10 inch chamber, you get a muzzle energy of 37.2 ft/lbs.

Increase chamber volume to 200%, muzzle energy is 38.7 ft/lbs - a mere 4% increase.

Increase chamber volume to 400% of the original, muzzle energy is now 39.6 ft/lbs - just 6% more muzzle energy, in practical terms you would never notice this increase without the benefit of a chronograph. In terms of filling the chamber though, you would certainly notice the difference, especially if filling by hand pump, and the shot would be much louder too.

This applies in your case, your barrel volume is much smaller than the chamber, this means you have two options:

- Keep the same barrel, but cut down on the chamber volume considerably. You'll get a tiny decrease in performance but much less noise and pumping effort

- Keep the same chamber volume but make the barrel as long as you can afford to make it, you'll get significantly increased power and less noise for the same pumping effort.

[jordzan]

Thanks for all the info jacks and Co. I just finished glueing up a barrel a little over twice as long so that should shoot alot better. Im really interested to see the difference first hand ... and properlly shoot my gun for the first time, its on tommorow .. inanimate objects beware!

How considerable of a chamber cut down could be done ? if the barrel is never going to be more than 3-4 feet is there a general guideline chamber size to not exceed ? i still have it stuck in my head a little that a chamber the size of a house will shoot a golf bowl around the world lol

With the GGDT program, is there any way of entering details for a sprinkler valve, or is it only for piston weapons ? not quight upto the piston stage yet, still trying to get a good grasp on it before i jump into making one.

[inonickname]

Easy. Have a look at these GGDT files of my next-gen Q-tip gun.

The first chamber has a length of a mere 5 centimeters, while the next has a length of 80. For 75 more centimeters of chamber, I get only 100 feet per second- it would be much less painstaking to mix up some NaOH + Al and use hydrogen, which bumps the fps up to 1400+ fps. Also, there's no way I could pump the larger chamber with my shock pump, and it would be extremely noisy, as opposed to the other which will have dropped the chamber pressure by 290+ psi, down to 100ish which will produce a much quieter report. In fact, the next chamber may get even smaller.

Just use the generic function to model a sprinkler valve. You may have a few issues with some numbers, but nothing too hard to work out.

[jordzan]

nice one inonick cheers mate. starting to get a good grasp if it now.

was hoping to ask, in the ggdt program under the chamber data section.
what do you they mean by inner and outer diam ??

i was figuring inner diam was how wide the tubing used for chamber is and then the outside was just used to determine thickness or something. but when you change the outer diam it drastically changes the volume in3 ??

[skyjive]

This feature is for coaxials and such, where your chamber cross section looks like a doughnut, with the middle bit not part of the chamber and the outer ring pressurized. In this case two diameters must be specified to calculate the volume. For most normal guns, the inner diameter is zero and the outer diameter is whatever the interior diameter of your pipe is.

[jackssmirkingrevenge]

if the barrel is never going to be more than 3-4 feet is there a general guideline chamber size to not exceed?

Generally you should aim for a 1:1 ratio as a compromise between practicality and power, the only reason to have a larger chamber is if you want to squeeze ever last fraction of a ft/lb of energy from your setup which is generally futile.

It's also worth noting the relationship between pressure and power - if you half the chamber size but double the pressure, technically it's the same amount of air but you get significantly more power.

Very nice paintjob btw, really neatened things up