WaterWar.net

I found a 5 pound dry chemical fire extinguisher cylinder on the side of the road. It is empty, you can see right into it. It's threaded internally. It is a little heavy, but I am wondering if it would be possible to make this into a water gun? Could/would you use a regular water-only fire extinguisher in a waterfight?

Hmm, I have considered it before myself, although they are so akword (<- figure that out, spell check wont help) that I wouldn't recommend using it in a competitive war. My dad has one of those, I fired it off the porch some and it was pretty fun, but it was so big that you couldn't even fit it in a backpack. Mine also leaks a bit.

It's pretty small... I mean, it's 30% smaller than say, a Vindicator.
Awkward.

If you can clean out the internals decently and fill/pressurize it after any necessary modification, it could work. Metal containers are stronger, after all. Do need to see how the internals look though.

There are no internals. Just an empty tank.

There's a valve, pressure gauge, and nozzle. If there's an intake tube, it works for liquids, but if it's all gas, it isn't needed.

No, no, and no. It's just an empty metal cylinder. There's nothing attached.

Sorry, I was referring to complete fire extinguishers. For just the container, you'll want to see if the threads are NPT, and if they are, you can attach some metal fittings and turn it into a high pressure water cannon. Otherwise you'll have to improvise.

How would I know if the threads are NPT? What is NPT?

Oh, that one is much smaller then mine. The one I have is around 4 feet tall and ways probably 10 pounds when empty.

NPT is a type of thread (National Pipe Thread) standard across the US. Best way to check is to take the extinguisher bottle to a local hardware store, find some NPT threaded components and test how well they fit into it's thread.

It probably will be standard NPT. Over here, even aluminium drinks bottles seem to have BSP (British Standard Pipe) threads (our version of NPT), making them great for NERF/water blaster PC's. At £1 per 1 litre PC it's a bargain!

NPT is probably less pervasive in the U.S. than BSP is in Britain. Drink bottle threads don't conform to anything, nor do any water blasters, which is a real pain. Hopefully 3D printing and scanning will be able to mitigate this problem later on.

Found another tank on the side of the road. This one is rated for 75 PSI and is roughly 1 foot wide and tall. I imagine it holds a gallon or two?
I was thinking, I could hook this up with a hose +ball valve. Screw it into my garden hose, open valve, fill er' up, close valve, and use it just like that?
I think I might just have to try it. The problem is that the thing is so wide, it would be very difficult to carry. It would be fun to make as proof of concept, but would probably be a stationary device. I would much rather have two narrower cylinders.
My main concern is that there will be enough pressure coming from my house (around 30-50 PSI I think) in order to give half-decent power.

I've made homemades of similar design and they work pretty well. However, you may have a difficult time keeping it connected to a hose with just a ball valve. I put a check valve on my homemades for hose intake. Adding a schrader valve would allow you to pre-charge it. Also, if you don't make a dip tube of some sort, you'll have to hold the tank upside-down for it to work.

If you want further info, I'll take a few pics.

This tank is weird. It has a Schrader valve (just like a bike tire) on the top end, and is threaded and sort of rusty on the bottom end.

Can anyone make sense of the label? I think it's some kind of water heater tank thingy. It has no electrical connectors, so maybe it is designed to provide air pressure to the water inside? Maybe it even has a flexible bladder inside? I am not sure how it works at all.

That's an expansion tank. It basically has a rubber bladder which separates pre-pressurised air and water so it can be used any which way up and won't need precharging every time. The bladder doesn't stretch it just rolls into itself as it empties. I used one in a homemade I made a few years ago, although that was an 8 Bar (116 PSI) tank.

As for the label, it looks like it has a max working pressure (i.e. max allowable pressure after precharging and filling with water) of 75 PSI. The 12 PSI should refer to the precharge pressure that it came with but that's probably changed by now and you can increase it above that if you want. I think they usually come charged with filtered Nitrogen rather than air so the inside doesn't rust, but you should be OK to whack on a normal pump to top it up, just avoid doing it on really humid days. Obviously 240F and 14F are three max/min working temperatures. Not sure what USG is though.

It's red because it's for heating systems rather than potable water (white tanks) versions rated to higher pressures. At that size it'll probably be an 8 litre tank, although that's usually the volume of the tank itself. You'll probably only get 5 litres of water in it as the listed volume is usually of the tank itself and not the bladder.

Thanks Andrew! Unfortunately the inside of the tank was rusted out, and someone must have taken it to the scrapyard by now.