You make some good points there badger...hadn't considered the inherent flammability of the materials we're dealing with. Also, to clear things up a bit, the amount of explosives in a cap is about a hundredth of what you were saying (about .2 to .3 grains). Still, the safety concerns you raise are vaild.
The gun that he is talking about, I believe, is the disposable six-shot party popper gun that is manufactured by Ja Ru Toys. We tried them out at the RHPS for the Transies. The package actually says 25 grains of black powder. I will check this tomorrow to make sure. There is a dollar store in town that sells them.
As carbon said there is almost no explosive in caps. I have already considered the things you all said.
I going into this have the intention of not making these more than toys. Also you would have to light like 200 caps to get anything of any danger especaly with pvc being rated to such high preasures and the party popper gun being made to do this. I am not trying to make anything more than what could be considered a toy.
I hate typing out wordy posts to get my point across, but I am wide awake right now and I have some free time.
Here is an example of simple physics and chemistry.
First the physics part of it. For this example, we are going to cut the grain count down the middle between Carbon's and my figures and say about 10 grains per cap. In the instance of the party popper gun, the powder load used is propelling a chamber full of either mylar confetti or mylar streamers, depending on what barrel is in position when you pull the trigger. When the confetti leaves the barrel, it fans out into the air and flies about 8-12 feet. We use another version of the gun at the RHPS and that is about the distances we get. The materials of the barrel chamber are simple plastics. The energy from of the cap's explosion is not nearly enough to destroy the chamber and alot of the energy is diffused through the confetti itself. Probably more than half of the energy is wasted when it filters through the spaces in the confetti, hence the short ranges. Please make a note of this fact. The gun is disposable, meaning it was not designed to fire more than one round out of each barrel, whether for cost or safety reasons. If you use a micro stefan, you, of course, will get better ranges since it is a more solid projectile. Most of the blast's energy will propel the dart since there are no spaces in the projectile for it to escape through. Though the chamber may still be able to hold up to the pressure of the blast, it is beyond the original intention of the dseign of the chamber. If you increase the amount of caps, you risk a breach of the chamber. The popper gun is most likely not rated to handle such stresses. If it was, it would cost alot more to make them and they wouldn't be found at dollar stores. They would also probably be reloadable. In addition, the extra force of those additional caps will propel the dart farther, but at the added risk of causing possible injury to anyone hit by it. If you were to build your own gun to accomplish this, you may avoid the problems of the gun exploding, but there is no guarantee. PVC's pressure rating is based off of fluids, not gases or explosive pressures. While it may withstand it at first and maybe even after a couple of shots, it will lose its structural integrity after a while and become a hazard.
Now for the chemistry end of my observations. Let's once again look at the original gun specifications. The materials are a single-shot cap which is the propellant and produces a tiny explosion, the shell casing or chamber which is made of plastic, and the mylar confetti. The amount of fire and heat produced by the gunpowder is quickly comsumed. There is not enough heat or flame produced to cause the chamber or the mylar to melt. I am not sure of exactly what type of plastic is used in the popper gun, but I do know that the melting point of mylar is 500 degrees Fahrenheit. I looked it up to make sure. If you were to build a homemade gun, you are adding an additional factor into the reaction area. You are adding, most likely, a sealant to adjoin the sections together, most likely epoxy. So if you were to use, once again, more caps to launch your dart, the caps would explode and their checmical reaction time would still be minute, but the possible added fumes from the epoxy, plus the fact that epoxy changes the checmical structure of the PVC slightly during the bonding process, could very well enhance the explosion. Since the dart will not let barely any of the energy and force of the explosion escape through itself, unlike the confetti, and add into it the fact that the integrity of the PVC has now been comprimised by the epoxy's sealing characteristics (in case you didn't know, epoxy "glues" things together by melting and fusing them on a chemical level), you have a greater chance of an explosion. The epoxy fumes can make the cap explosion increase in magnitude.
I am done with my rant. You can do whatever you want. You have the freedom to completely ignore me. You can heed my educated warnings and steer clear of your idea. I just hope that I don't see on CNN one day about how some guy blew his hand off with a homemade Nerf gun.
Piney, I seriously need a brain enema now. I really had to think on this one and now it is after 1 A.M. here in NJ. I didn't mean to reiterate my previous comment in novel format, but I felt that it was necessary to try and help someone see the light.
God I feel old.