Intro: Build a Pneumatic Air Rifle
I have always enjoyed making pneumatic potato guns. There's always fun to be found with a gun that shoots anything you can fit in the barrel. So I've decided to post one of my more recent builds of a small caliber potato gun. This is technically my first instructable since my mini-fridge was just a slideshow, so please bear with me. Plus, I feel I have to redeem myself from that mini-fridge since it ended up all over the internet.
Generic Disclaimer: I'm in no way responsible for anything anyone decides to build/create/destroy with anything from this instructable. Don't even try to build anything related to this if you don't know exactly what you are doing. If you are unsure about anything in a build process with compressed air involved, seek help of a qualified individual or just don't attempt it at all. Be responsible and use common sense :)
A little background info.... The idea to make this particular gun design was kind of a spur of the moment idea. I had some small pieces of PVC laying around and an extra sprinkler valve so I decided to go at it with what i had(save some money too). Most of the potato guns I make are usually larger diameter barrels (1" - 4") but this time I wanted to go small with it. The diameter of the barrel I decided to use was 1/2" and the air chamber was 2". I wasn't sure how well this would perform but it seemed worth a try. I will try to be as specific as possible in the build process. If you don't like part of the design or have any feedback, please let me know. I do like to hear peoples opinions, good or bad.
Here's a materials list of the items I used in my version. Depending on the size you wish to build, this list can change dramatically.
This is not a complete list, these are just some of the main parts.
1/2" PVC Pipe
2" PVC Pipe
3" PVC Pipe
2 - 3" PVC Couplers
1 - 3" PVC Cleanout with Plug
1 - 2" PVC Cleanout with Plug
2 - 2" PVC Coupler
1 - 2" to 1"threaded bushing
1 - 1" by 1" threaded pipe
1 - 24 Volt Sprinler valve
1 - 1" threaded to 1/2" slip Coupler
4 - 10" Hose Clamps
2 - 4" Hose Clamps
1 - Cheap Airsoft Gun
1 - SPST Pushbutton Switch
3 - 9 Volt Batteries
4 - 9 Volt Battery Terminals
Miscellaneous Pieces of Wood
You'll also need some pvc cleaner, pvc glue, expanding foam, liquid nails, and miscellaneous building tools.
Also, sorry if some of the pictures are crappy. The best thing I had to take pictures with during the first part of the build was a crappy cell phone.
Step 1: Air Chamber
I started with building the chamber to hold the compressed air. I build my potato guns by "eyeballing" it. So I don't have any exact measurements for this. Plus this is a chance for people to all have something different in case anyone actually decides to build something like this.
For the chamber, I used:
1 - 2" PVC Cleanout
2 - 2" Couplers
1 - 2" Slip / 1"Threaded Bushing
There's only one way those parts can be put together so it's a pretty easy step. Make sure to be generous with the cleaner and glue. You want to definitely make sure that you are getting a good air-tight seal on all parts of this project. Always wait the recommended curing time for the glue you are using. I've had caps pop off on parts before and it has the potential to be very dangerous.
The purple on the PVC is the primer in case anyone is wondering.
The next part has to do with the end cap. There can be many different ways and connectors for the end cap but it's all the same basicprinciplee. I decided to use a quick-disconnect fitting on the back of mine. I chose this because this can help with refill times (just keep it connected) and I don't like using tire valves. This is most likely the place where a leak will form in the air chamber. It can be challenging to get a good air tight fit on this. I found a pretty easy way to fix this by heating up the couple part for the inside of the cap to a very hot temperature. It screws on rather loosely (it expanded) but it melts the PVC on the inside and gets a good seal. There are possibly better ways of doing this but this way works for me.
After letting my glue cure it was time to put the cap in place. I didn't use plainTeflonn tape on the threads. I used tape specifically designed to seal gas. This tape is a little thicker and in my opinion has been more durable and reliable than plainTeflonn tape. But I'm sureTeflonn tape will probably work just fine.
At this point, the chamber can be considered done. I would recommend testing it before using it though. You can just put a 1" plug on the other end of the chamber and dunk it in a bucket of water to check for leaks. Be very careful with this also. If the chamber blows up in a bucket of water, it can be a very big explosion. My chamber was good andair tightt. I tested it up to 100psi. So I'm good to move on.
Step 2: Carrier
The guts carrier. It's self explanatory in a way. This step is just building a shell to hold the chamber, valve, electronics and barrel in(the guts). This is certainly not needed for a basic potato gun but it can help the looks out a lot. I'm using a 3" diameter pipe for the carrier. The 2" pipe with fittings fits with just a little room to spare.
The first part is to figure out where to cut a hole for the air valve. I'm using a 24Volt sprinkler valve for mine. Depending on the valve you are using, this part can vary or not be needed at all. I didn't worry about making the hole look good since it will be covered up in the final design. The sprinkler valve ended up barely fitting in the 3" pipe. I got lucky this time I guess.
Step 3: The Barrel
The barrel I used was a 1/2" one made of PVC. The cool thing about using the threaded sprinkler valve is that I can change the barrel if I need to. It's just a pipe with a few fittings on it to get it to a 1" threaded. You can up the barrel diameter since it's interchangable but one thing to note is that there is only a certain volume of air the chamber can hold. The larger the barrel, the larger the barrel volume. Which means it will take more air to shoot something at the same velocity. The length of the barrel is also something I kind of eyeballed. I wanted it to stick out of the carrier but not too much. It's roughly 18" long.
Step 4: Combine the "guts" and the Carrier
When piecing together the barrel, chamber and valve inside the carrier you have to get the valve in place first. Once the valve is in the hole made for it, you can insert the barrel and chamber in their respective ends and screw them tightly in to the valve using thread tape. Once you have everything put together inside, it will be very loose and roll around inside. The best thing I've found that works for this problem is using expanding foam around the valve and chamber. This works well because it is not permanent, very easy to use and light weight. I chose not to use it around the barrel since mine is interchangeable.
Step 5: Cover the Valve
The valve is a very durable piece but it is something you want to make sure to take care of. I try to always cover my valves I use. This again can make it look better but it can also protect the solenoid from any damage. There are many ways to cover the valve if you choose to. I built a box and cut it so it would fit exact with the 3" pipe. To attach it, I used hose clamps to just clamp it on. I did clamps because they can secure the whole box instead of like screws and bolts which can secure it only where they are placed.
Oddly enough this was the hardest part of the build. I just kept having problem after problem of the wood breaking or I cut the wrong spot. Luckily I got it done though and secured. I had built the box a little too big so the hose clamps I had wouldn't fit. Rather than rebuild that demon box I just daisy chained some smaller clamps to make it work. I may go back and change it later.
I didn't secure the box tightly because inside is where the batteries and wire will be contained. That will be put in later on so I just screwed the clamps down enough to keep it from moving around.
Step 6: The Handle
Most potato guns I see on the internet Just use PVC pipe as the handle. I've done this before but I think it looks a little silly and just doesn't feel right. So I started using pistol grips for my handles. This can be done with a cheapo airsoft gun. I get them at my local flea market for about $2.00 each. That's cheaper than it costs to buy the pvc fittings to make a pvc handle. The main thing to look for is how sturdy the plastic feels.
The first part is to get the slide off the gun. This can normally be done by removing a few screws but if all else fails a Dremel will always work. Once the slide is off, you want to reinforce the front part of the gun by gluing something round inside the groove to keep the clamps used to secure it from smashing the gun. I've used wooden dowels, bolts or even the metal weight that sometimes comes in the airsoft gun for this. All of these worked fine.
As for the trigger, I used a SPST momentary pushbutton switch I got from allelectronics.com. It fit perfectly in to the hole left by the orginal trigger. All I did was screw the switch in with the provided screws.
Mounting it is very simple. I used hose clamps again (I like hose clamps) to secure it to the gun. The tip of the airsoft gun went right up to the box covering the valve. This is so I can run the wires though a drilled hole for the switch. It goes straght from the box to the handle without having any exposed wires.
Step 7: Power Supply
The sprinkler valve I used is rated for 24 Volts. The easiest way to get close to this is to just wire three 9 Volt batteries in a series configuration. This can be done by soldering but I just used some 9 Volt battery clips I also got from allelectronics.com. The clips work if you use them with the batteries side by side. It's a tight fit but it works.
Since this is a basic solenoid on the valve, polarity doesn't matter. Just hook one end of the batteries to one side of the switch and the other end to a side of the valve. The of course complete the wiring by hooking the free side of the switch to the free end on the valve. You can attach the batteries on the inside with glue, tape, velcro, zip ties....whatever you want really. You just want to make sure they are secure.
Note: You can use whatever kind of batteries you want for powering the solenoid as long as it works. I've used anywhere from a lot of spare AA's I had to a sealed lead acid battery. As long as it will pull the solenoid it should be fine. Rechargeables are great too. You can even build in a chanrger to the valve cover so you can just plug it in without taking anything off the rifle.
Step 8: Paint
Painting of course is an option but it usually adds to the over all look of the device. I chose to paint most of this black with a few yellow spots on it, mainly because those were the only 2 colors I had (I was limited on this build with what I had to use). The black paint was done half gloss and half flat since I ran out of flat part way through. Looks odd but it's better than bare. Another thing to note to this not-so-good paint job is that I'm in Florida and the humidity on the day I painted this was 100% and had been for the 2 days prior. It took 2 days to dry out in my garage and it has a kind of rainbow oily look to it at certain angles.
Step 9: Attachments
For this gun, I've only made one addition to the barrel. I added about a 1 ft section to it that I can screw on and off. It appears as a silencer but it is not. It's a cover, essentially, for the ammunition. The main ammo I chose to use was .50 caliber black powder rounds (minus the black powder of course) since they are built to be shot and readily available at Wal-Mart. But one of the more fun things to shoot from a pneumatic cannon is an arrow in my opinion. I wasn't planning on modding this for arrows but I couldn't help it. I didn't have enough pipe to make the barrel longer and I didn't really want to use larger pipe. This actually turned out better. With most of the barrel having less volume, there's not as much expansion the air has to do to cover the same distance in the barrel with the same pressure. If that makes sense...
Step 10: Fin
I haven't gotten a chance to fully range check this as I live in a relatively populated area. I'll try to update if I can get a chance to test it. I shot a few things with it like card board boxes and foam targets. It works ok. One thing to note is that if the .50 caliber rounds didn't pierce the target (too low of a pressure or odd angle), it had a bad ricochet. The arrow on the other hand worked like a charm every time :)
The good thing about this is that it is scalable. I'm sure a larger version would work much batter but you need to lay down some more money for that. Overall I'm pleased with it. Any questions or comments, please let me know. This was my first instructable so I'm hoping to do more in the future.