Couple beginner questions I can't find...

[erager09]

I'm new to the forum and have a basic question on pneumatics.

I was searching for a bit today but everything kept going to airsoft guns etc. I want to build a potato gun...nothing super fancy. But i have a multitude of HPA tanks from my paintball guns. They are steel tanks at 3k psi and i believe internally regulated to 500psi coming out the tank. I want to try and use these as an air source to launch the gun.

I have thought about the HPA tanks connected to an inexpensive paintball gun reg (for adjustment) and that attached to a simple ball valve. Do you think this will release enough air to adequately launch a potato? The steel braid line between the tank and reg doesn't have a very large internal diameter which might limit airflow. But that is pre-regulator, so I'm thinking the 500psi through the smaller tube might be adequate since its gonna be lowered to 100ish.

If there is a thread I just didn't seem to come across it in my searches.

[jackssmirkingrevenge]

Rather than a ball valve, why not go for a burst disk? A tank made out of metal fittings would happily take the 500 psi directly from the paintball tank (check the exact output pressure - Ninja tanks for example can be adjusted to give 450 to 850 psi) and what you'd need is a length of pipe for the chamber, a union in which you could sandwich layers of material such as foil and a length of pipe as a barrel. Open flow to the chamber, pressure rises and in a few milliseconds, *blam* and smiles all round

What budget/dimensions/calibre did you have in mind?

Do you intend this to be a fixed "mortar" type launcher, or something handheld/shoulder launched?

to launch the gun

Stick to potatoes, it's cheaper

[erager09]

Definitely handheld/shoulder fired. Mortar is just too big. I have a 2dr coupe and i'd like to be able to toss it in the car with a seat reclined and not have much trouble transporting it. Budget wise...if it worked good I'd say $100 to $150 bucks. I'd rather keep it lower but if its gonna be 150+ i'll just do a pvc combustion gun and play around with that.

Looking up burst disks that sounds interesting. From what i could see it in a diagram i saw it looked as though you could rig it to be breech loading which would be great. From what i saw you open the "union" place the burst disk and close the union so would that also be an opportunity to load the potato?. Just how noisy are they usually? Don't want unnecessary attention from the po-po cause i live in town but a field butts up to our property..

Size wise I'm guessing an average potato. I want it to be really easy to just go buy a bag of russets and have fun. I also thought about golfballs but lets stick to potatoes. (changeable barrels would be perfect though...)

[jackssmirkingrevenge]

If you can reg down to 150-200 psi you can keep things pneumatic and cheap.

Looking up burst disks that sounds interesting. From what i could see it in a diagram i saw it looked as though you could rig it to be breech loading which would be great. From what i saw you open the "union" place the burst disk and close the union so would that also be an opportunity to load the potato?.

Yep, theunionallows you to disconnect the barrel from the breech.

Just how noisy are they usually? Don't want unnecessary attention from the po-po cause i live in town but a field butts up to our property..

A medium bore pneumatic with a good valve is pretty loud, you can keep it low (and efficient ) by keeping the barrel long and chamber short.

Size wise I'm guessing an average potato. I want it to be really easy to just go buy a bag of russets and have fun. I also thought about golfballs but lets stick to potatoes. (changeable barrels would be perfect though...)

A quick search of the forum will dig up what pipe size is ideal for golf balls.

Basically, this is the design I am suggesting:

[erager09]

Well im off tomorrow and im going to swing by harbor freight to look at some of these parts... hoping maybe theyll have the union piece so i can see it in person.

My only questions at this point are 1. Place to get a supply of burst disks? Saw soda cans work ok for about 90psi shots. 2. Material needed to safely shoot? PVC at 100-150 psi or is that in the realm of metal piping? What keeps the blowgun handle from getting ejected instead of the tater...? Or are the ends of the guns threaded? Could just drill and thread a hole into the pressure chamber if thats the case.

Basically I'm just trying to do this so i don't end up breaking my wrist or injuring someone if s.h.t.f.

[Hubb]

Do you plan on using a chamber? If not, why? A chamber would solve the problem, provided you have a proper regulator.

Place to get a supply of burst disks? Saw soda cans work ok for about 90psi shots.

Davidvaini uses foil tape. I've used it as well. It works great for this. I've never used a piece of soda can for a burst disk, so I'm not sure what type of pressure it would hold. But foil tape would work, is easier to work with, and can be layered to provide greater pressure easier.

Material needed to safely shoot? PVC at 100-150 psi or is that in the realm of metal piping?

The pressure of the PVC is going to depend on the size and type of it. DO NOT use anything less than Schedule 40 pipe. Also, make sure the fittings are properly suited as well, for that is where most launchers fail to begin with. And, if in doubt, use metal pipe.

What keeps the blowgun handle from getting ejected instead of the tater...? Or are the ends of the guns threaded? Could just drill and thread a hole into the pressure chamber if thats the case.

The fittings should be threaded. You could just drill and tap a chamber but, honestly, you should go with fittings instead. Safer that way.

[jackssmirkingrevenge]

Further to Hubb's advice:

1. Place to get a supply of burst disks? Saw soda cans work ok for about 90psi shots.

First of all, a given material will fail at a certain pressure for a given diameter. For example, withthis 6mm bore project, a single layer of ordinary stationery tape was enough to hold 400 psi. I am currently using photo paper for my hybrids, two layers for a 3/4" bore will happily hold at least 200 psi. A bit of trial and error will give you the right material and number of layers, the important thing is that your chamber is strong enough to hold the pressure should the disk not fail.

2. Material needed to safely shoot? PVC at 100-150 psi or is that in the realm of metal piping?

Certainly within the realm of pressure rated PVC - make sure you check out the rating and work within the limits specified. Again you might want to consider metal fittings to work with the 500 psi feed from the tank directly, a small metal high pressure chamber will yield better performance than a large low pressure one, have a look at these calculations. It might be a good idea to download and useGas Gun Design Toolbefore actually purchasing any material, in order to make a virtual model of your launcher giving you approximate performance for given pressure, dimensions etc.

What keeps the blowgun handle from getting ejected instead of the tater...? Or are the ends of the guns threaded? Could just drill and thread a hole into the pressure chamber if thats the case.

It's assumed the endcap is held on with PVC solvent welding, plenty of advice on the forum. You can drill and thread the endcap and epoxy a 1/4" nipple to keep the blowgun in place which should work well enough for lower pressures, but the better option would be to buy a threaded endcap and get to the blowgun's thread through reducers.

[erager09]

Ok so I purchased materials for a normal pvc combustion style potato gun. Bought all Schedule 40 pvc. After assembly I was looking at the codes printed down the tubes just out of curiosity. The barrel (2") says Sch 40 and then has a psi rating at 73 degrees F. The combustion chamber does not have this. It does specifically say Schedule 40 but it also says "not for pressure t.p." What does this mean? I specifically asked for schedule 40 pvc at lowes and thats what the guy got me. He even asked "Is this for a potato gun...?" And i told him yeah and he said thats 90% of the kids coming in the pvc section. (I'm 26 but he was older). I have yet to fire it and am gonna throw a heavy duty blanket over the gun for the first few test fires just incase. Also bought bulk rolls of duct tape to wrap the chamber just incase it were to splinter down the road. Just different things I saw in here that made sense to me safety wise.

[clemsonguy1125]

Can you post a picture of the parts? It will let us double check the parts are pressure rated before you glue them. If it says not for pressure, then it for drainage and is not pressure rated and cell core. Theres a myth that all sch 40 is pressure rated but thats incorrect.

[Gun Freak]

Also bought bulk rolls of duct tape to wrap the chamber just incase it were to splinter down the road

You shouldn't even need that. If it says schedule 40 and has a pressure rating, you will be fine. The stuff won't explode until over 1,000 psi if you know what you're doing.

[clemsonguy1125]

But is it says not for pressure t.p, I wouldnt trust it, it should have a pressure rating on it.

[erager09]

Yeah everywhere I looked it just said use schedule 40. It's already glued. My brother asked about it but the guy said its schedule 40. I was under the assumption its the same stuff but I've been sitting here at work wondering so I wanted to ask.

[Gun Freak]

Never trust the Lowes guy

[jor2daje]

I'm not sure how many times this is gone over but sch40 isn't a pressure rating it's refers to the thickness of the pipes walls.

[erager09]

Posting on my phone so I'm having trouble getting the link right

So my gun is worthless pretty much. Someone else was talking in a thread that ya just use schedule 40 and your good for the normal hair spray fueled potato gun. Glad this wasn't a $200 screw up.