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Blow-dart combustion gun **updated slight mod.

Built a combustion cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: POLAND_SPUD » Sun Jul 20, 2008 9:44 pm

hey these darts are great I haven't realized that they are so good... I add them to my list 'things I want to have'

@radiation nice gun but consider switching to pneumatics for more power
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Unread postAuthor: Radiation » Sun Jul 20, 2008 10:24 pm

skyjive wrote:In your damage pics I couldn't help noticing that the darts are sticking out of the wood at all sorts of angles. In my experience with darts, this generally means that they were fired from close range and were ballistically unstable, i.e. were starting to tumble in flight but didnt have time to tumble too much due to the short range. Alternatively, they could have been fired at the target from different angles or bent on impact. Have you tried shooting these projectiles at long ranges? If so, do you know if they go straight? Those darts look very deadly if they could be fired straight and accurate at long ranges.


Very observant! yes indeed these darts were fired at very close range 10 or so feet, solely for the purposes of this post. As a result, as you stated, they tend to angle upon impact. I have fired from distances of 50 to 80 feet and it is extremely accurate (my aim is another story entirely) I was hitting soda cans at 30-40 feet, the gun shoots where you point it very reliably, I haven't quite mastered my accuracy. the darts do stabilize at these distances, my homemade darts lack the aerodynamic refinement of the cold steel darts and tend to slow to the target over distances.

Deadeye: The fan pic is a bit deceptive, it is actually about a quarter of the way in the chamber from the ignitor side. This leaves 3/4 of the chamber free and clear so nothing is obstructing the combustion into the barrel.

The fan is pointed away from the ignitor into the chamber. I find this has a few benefits. One, the butane goes in and you don't lose a lot of residual gas. Two when you "vent" the chamber it pulls fresh air in without necessarily having to have a clear barrel (this is important because with the revolver once I shoot I advance the next dart).

As you can see I have gone with a fan that is much smaller than the chamber circumference. Through experimentation I have found that a small fan works more efficiently in a big chamber than a big fan in a big chamber. Big fans tend to move air very well in it's general vicinity, but on a longer chamber (say a chamber that's more than twice the diameter) I've found that my guns just don't seem to be as powerful as they should. Putting in a smaller fan solved this problem. My thought process is that this is because air circumvents around the back of the fan more readily than it would for a bigger fan.

As far as venting the chamber quicker I like your idea. I have used a ball valve specifically for venting a chamber in some of my larger designs. I thought about putting a ball valve on the other end actually. Between the Chamber and the elbow I could put a T connector. one end from the chamber, the other end toward the barrel. The third end would be the ball valve that you just open to vent. Assuming I would go to a propane metered system I could add a ball valve at the other end to vent from one side to the other. This would only add a few bucks to the price and remain relatively simple.

Poland_spud you are right I am very interested in pneumatics, but I need some time to digest all the info on them.

I don't know I think the knowledge base in this group is so vast my head is spinning! So many possibilities, so little time and money. :D

Thanks again to everyone for the suggestions and comments!
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Unread postAuthor: Deadeye » Sun Jul 20, 2008 11:16 pm

Ah!

I see, said the blind man.
The photo lead me astray.

You know my first instinct was to suggest a "T" instead of elbow with the ball valve beyond but it would require a "draft" which as you said would need to be another ball valve at the other end, (or in your case open a valve & a cap) which I thought would be a deal breaker. I liked it because again you could house all your hardware beyond the ball valve in the "safe zone" In reality it would be unique because you are only "refreshing" the chamber and not the barrel which in most (ideal) cases is double the chamber size... again unique. but one more step of labor with the two ball valves instead of one.

In most cases everyone blows all the way through the barrel.
I really wasn't understanding your "loader" enough - but now get that you don't have that option with the reload in the path, so it is more important to have an in & an out for the air, beyond the dart path which at best would be blow-by.

I also almost get what you are saying with the fan sizing but haven't reached a conclusion. I put in a slightly smaller (more powerful) fan and now get a sort of blow back out the ball valve (back side) I didn't get before. Frankly I'm not sure if this is good or bad - it could help to sort of intake and exhaust at the same time or it might just be reducing the forward pressure - I believe it has to do with the fan circumventing like you say but I can't calculate the good/bad.

I am digging your ammo and much appreciate the clarification brought about by skyjive's questions.

Sorry to deluge you with posts, but as you can see yours is a bit different and interesting.

Keep up the good work.
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Unread postAuthor: Radiation » Sun Aug 03, 2008 11:10 pm

Ok so I took everyones advice and decided to go full on propane system. Here is the result! Did the math (three times just to make sure), did the conversions, did the modification and the thread tapping as well as about 6 trips to and from various hardware/tool stores and it all was worth it! By the way, the "slight mod" part was meant in jest. :D

Image
(like an amateur I dripped primer on the chamber I was so mad!)

Full propane injection system with breech loading setup. I have some bad news, the revolver setup no longer works with this system. It turns out that propane injection is significantly more powerful than my previous setup up. The result, the poorly assembled revolver was blown apart, not damaged, but in need of severe modifications which I am currently working out. Any way I came up with a pretty decent solution for rapid reload with this breech setup.

Image

The T connector is on a swivel so you can pull on the barrel to release it from the elbow, it then swings to the side so you can quickly and easily load your next shot.

Image

I made a new 180 degree elbow and modded the valve to be smaller on the far left to help vent the gun between shots. I still unscrew the cap for the ignitor to vent as well. I don't really want to change the ignitor system as this is supposed to be my small gun (I'll post the big one someday I swear, it's just not ready for prime time yet!)

And now damage pics!

Image

These are the nail darts from my original post. With propane injection they go so far in I can't get them out with pliers. All the nails are forever embedded in this wood. I tried hammering some of them out to the other side, but it was way more work than it was worth.

I am extraordinarily pleased with the results from the propane injection. Many thanks to those who suggested it! A BIG thanks to burntlatke.com for their very in depth and thorough explanation/diagrams/calculator tools. It really put this project over the top, both in performance and budget. :oops:

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Unread postAuthor: starman » Mon Aug 04, 2008 3:24 am

Slight mod indeed! Metered propane and air vent makes a big difference doesn't it. Really clean effort here. I like your breech load system...simple but effective.

The propane tank is mounted "mostly" upside down relatively speaking so you're likely to "flood" your meter with liquid propane under some conditions.

Also, there doesn't seem to be a defined way to hold it. Holding it during firing looks like it will be a little awkward...kind-of have to just cradle it in you arms and reach for the ignitor.

Maybe you can break out a little paint job to cover the primer stains. Sand it first and use Fusion plastic paint. Overall, you did a neat job on this and I find the whole combustion dart gun concept pretty nifty... :wink:
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Unread postAuthor: Radiation » Mon Aug 04, 2008 6:19 pm

starman wrote:Slight mod indeed! Metered propane and air vent makes a big difference doesn't it. Really clean effort here. I like your breech load system...simple but effective.

The propane tank is mounted "mostly" upside down relatively speaking so you're likely to "flood" your meter with liquid propane under some conditions.

Also, there doesn't seem to be a defined way to hold it. Holding it during firing looks like it will be a little awkward...kind-of have to just cradle it in you arms and reach for the ignitor.

Maybe you can break out a little paint job to cover the primer stains. Sand it first and use Fusion plastic paint. Overall, you did a neat job on this and I find the whole combustion dart gun concept pretty nifty... :wink:


Thanks! I actually haven't had any problems with the way the propane tank is mounted. I hold it vertically (actually about 45 degree angle downward) when loading a new dart as well as metering the propane, so maybe that's why it's working ok. As far as holding it I usually shoulder it to my right arm and the ignitor is right their for my pointer finger to press. I use my left hand to steady the now 20+ lbs gun. It hasn't been a problem yet, but I have thought about some handles. I may paint it, but that has taken a back seat to modding the revolver.

I forgot to mention sort of a funny quirk with this setup. The metered propane has the tendency to push the dart out of the barrel if the system is completely sealed. It actually shoots the dart a good 10-15 feet just from the little bit of psi in the meter! So I have to leave the barrel valve open and then close at the same time as I open the meter in order to allow some air to escape rather than push the dart. kind of funny when it happens. :)
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Unread postAuthor: starman » Mon Aug 04, 2008 6:41 pm

Radiation wrote:The metered propane has the tendency to push the dart out of the barrel if the system is completely sealed. It actually shoots the dart a good 10-15 feet just from the little bit of psi in the meter! So I have to leave the barrel valve open and then close at the same time as I open the meter in order to allow some air to escape rather than push the dart. kind of funny when it happens. :)


Gives you an idea of the power of just a little extra pressure doesn't it. I would use this as an opportunity to make your darts fit a little tighter. You'll notice serious performance improvement.
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