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Bull Pup Rifle In The Design Stage

pump action - possbly combustion based

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#1 boltsniper

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Posted 09 March 2003 - 12:04 AM

OK. After a rededisgn of my trigger system for the bolt action rifle I was able to get about 25 shots per cylinder of C02. Each shot attaining a distance of about 150 feet. The scope is accurate until about 60 feet. Much beyond that. Drag on the dart starts to veer it off course. But you can still hit within a foot or so from 120+ feet.

Using what I`ve learned with the bolt action rifle, I`ve started work on a rifle of a bull pup design. This setup should shorten the length of the gun by about 18 inches. I intend to use the same shell system I have now in the blt action rifle. I might modify the shells a bit to accomodate the new action of a bull pup setup. The ejection port is going to be ahead of the magazine which might complicate things. I think I have all the kinks worked out of the action though. The new rifle will fire the same 5/8" stefans that the bolt rifle fires. It will also have the same size barrel and a interchangable silencer. I hope to have something constructed for you guys to see soon.

AS for propulsion for this rifle. The C02 works great in the bolt rifle. It is however not very cheap to operate at 50 cents a cylinder it adds up fast. I`ve been experimenting with hair spray and other fuels for a combustion based propulsion. I've made a prototype pistol to test this method. It is comprised of simply a single shell about 6 inches long with a cap on one end. Through the cap are 2 pins that meet inside the chamber about 0.1 inches apart. A quartz sparker out of a lighter is used to produce the ignition spark. My firsts attempts produced no combustion. I sued hairspray, WD40, and butane. Nothing would ignite. I have never made a potatoe gun so I am new to this concept. After spraying some hairspray in the barrel inserting a dart and leaving it for about an hour. I pulled this trigger and the dart went flying.......FAST and HARD. Now I expected a huge flame and a flaming dart to exit the barrel and act like a napalm round. I was so wrong. There was hardly any sign that there had been an explosion. A small pop and the dart went flying. Even more surprising was that the dart showed absolutely no sign of being exposed to a flame. There was no melting of the dart at all. I imagine this is due to the small combustion chamber used.

My idea for this method is to have shells with caps on the end already complete with iginition electrodes. The shells will likely be longer than the ones I am using now to allow for a combustion chamber. Prior to firing the shells will be sprayed with hairspray and loaded with a dart. The shells will then be loaded into the magazine of the rifle and fired. The shells will have to contacts on the back of them which will meet up with corresponding contacts in the breach of the gun. The trigger will be a simple quartz sparker that will send a spark inside the shell and send the dart on its way. The shell will then be ejected and a new inserted. The only real problem I am encountering is that the ignition of the gas is unpredicatble. I`m gonna do some mroe experimenting with this to see if I can get some consistent results.

I know you guys think I`m nuts for using combustion but the amount of gas used is about 2 cubic inches. Potato guns use combustion chambers int he range of 2 cubic FEET. Since the dart does not ignite I am inclined to give this a shot. Let me know your thoughts and if anyone has had any experience with this sort of propulsion I would appreciate the advice.
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#2 Zero Talent

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Posted 09 March 2003 - 02:27 AM

This actually seems quite interesting... So, you'd rather have this than, say, a two-stage trigger that sprays the hairspray, then proceeds to ignite it? And you expect no problems with the O-rings, then, from the results of firing a dart? You never know what the results might be, with the constant combustion, but even then, it's definitely worth a try. This could be very interesting. I'm not a fan of commercial propellants, such as CO2 or fuels, but I would like to see the results you get out of this. Good luck!
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#3 Cadmond

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Posted 09 March 2003 - 03:08 AM

I'm going to go with zero on this, and say that a 2 stage trigger that inserts hairspray then ignites would make much more sense, but I see why you're doing it.. or I hope I do. The fact that you can't get the hairspray to ignite right away is pretty odd... I dunno.. the only thing I can think of is add some air, but that doesn't make sense because it combusts later on.. I'm also fairly suprised that the dart showed no sign of being exposed to flame, but I guess it makes sense that with such a small combustion chamber, the dart wouldn't have much time to do so. I'm not really seeing the lighter sparker, do you have to hook it up to each shell individually, or .. something else? Hm.. if you did go with a 2 stage trigger, the gun might not have as much range, unless you seal the dart into the chamber, or seal a shell to the chamber. Anyways, good luck
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#4 Zero Talent

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Posted 09 March 2003 - 05:13 AM

Hm.. if you did go with a 2 stage trigger, the gun might not have as much range, unless you seal the dart into the chamber, or seal a shell to the chamber. Anyways, good luck

His charged shell method is much simpler, but he'll have to do more work, and use a tad more money, per shell. No biggie, wires aren't gold. Well, some might be, but I'm tired.
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#5 merlinski

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Posted 09 March 2003 - 12:21 PM

Cadmond's post just reminded me of something I forgot to post on NC about this gun.

The reason its not igniting is that there is not enough air. After about an hour, some (probably most) of the propellant dissipates and clings to the sides of the chamber. That leaves a much lower concentration in the actual chamber, and a much higher air to fuel ratio. This would also explain the fact that you aren't getting the muzzle-flash jet of flame. I'm guessing that your putting too much fuel in the chamber. Try a much, much quicker spray of whatever your using.

Zero, do you still check the NC boards?
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#6 Cadmond

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Posted 09 March 2003 - 12:51 PM

um.. hooray! Did I get it?

And yeah zero.. the charged shell would be simpler for 1 shot, but for multiples, it would probably be the same difference for shooting multiple darts out of a 2 stage chamber. Erm... maybe not..
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#7 Langley

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Posted 09 March 2003 - 02:37 PM

How long do the shells really keep? I mean, could you spray and load the shells and have them still work, say, five hours later? twenty five? Because if you have to wait so long for the damn things to fire, then making shells is going to have to be a day-before-the-war kind of thing. You're not going to want to do this an hour before an actual nerf war.
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#8 Frenzied fury

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Posted 09 March 2003 - 03:24 PM

I know very little about combustion weaponry, but I'm thinking that the time it fired at first is because you didn't let the hairspray vaporize properly. If you put it in, then cover the openings and shake it up for a few seconds, then it will vaporize and ignite better. Just an idea.
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#9 Zero Talent

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Posted 09 March 2003 - 04:52 PM

Merl: Occasionally. It's a bit stagnant right now, though, and I'm sort of partial to this board.
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#10 boltsniper

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Posted 09 March 2003 - 07:45 PM

I do beleive you`re right about the fuel air mixture in the chamber. I think the chamber is jsut too small to get the proper fuel/air mixture initially. Switching to a larger chamber (say 1-1.25" PVC) may work better. Having a chamber like this will also lend itself to a 2 stage trigger setup like you guys have been talking about. I had actually thought about using a system as you have described but through it out in light of the added complexity of the system with regards to the trigger. But thinking a little more into it I think it may be the way to go. Using a chamber installed in the gun will allow for the use of the same shells as the bolt rifle uses. Common parts are always good, and common ammo is always better. When I first brainstormed this idea my first thoguht was a separate combustion chamber as part of the gun. First ideas are always the best....I should know that by now.

The range shouldn`t be affected by separating the chamber and the shells. Using o-rings to seal the shells with the bolt and the barrel as in my bolt rifle should work jsut as well in this case. I don`t foresee any problems with the o-rings and the heat. The flash is so fast that nothing ever gets hot and nothing is exposed to flame long enough to be affected.

Having a separate expansion chamber will cause some issues to arise withe the bull pup design. The chamber has to be loacted behind the shells preventing the mag from being in the absolute rear of the gun. This isn`t a major concern though...it will only increase the guns stock by say 4 or 5 inches behind the mag. By using a bull pup setup i`m hoping to get the guns overall length dwon to about 2 feet without a silencer. My bolt rifle is 39 inches long without the silencer (46 with the silencer). This isn`t too bad but it can be hard to maneuver with if you are trying to shoot on the move. I`ve smacked the wall a few times. The bolt rifle has a 17' barrel (25" w/ silencer). I`m hoping on the bull pup rifle to actually lengthen the barrel a little. The new rifle will still have the optional screw on silencer.

I do beleive that combustion is a viable form of propulsion and I am going ot make it work.......... ;) I`m not very found with commercial propulsion either Zero. If I could get the power and low prime time with air without a massive system, I would definitiely go with that. But at a buck fiftty for a 14 oz can of fuel...........the price isn`t too bad. C02 adds up fast at 50 cents a cylinder. that is part of the reason I started this new gun. I`ll keep you guys posted
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#11 merlinski

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Posted 09 March 2003 - 09:49 PM

Merl: Occasionally. It's a bit stagnant right now, though, and I'm sort of partial to this board.

Ok, because I have an idea I'd like your opinion on (text copied from NC):

I've been thinking for a while about a system of using shells that contain the method of propulsion of the dart. The best thing that I've come up with is using springs, but that would mean long shells that would be harder to handle and would jam easier. I thought that the best system might be one in which the gun contains the propulsion method, but doesn't need to be re-primed. I sorta stole the idea from airsoft, which uses motors to pull back springs. I think that if it would be possible to create a system where a motor pulls back the spring, stops there, and then starts again once the spring is released, you would have a reliable continued-firing system that would work for the length of the AA or whatever size batteries that your using. The other thing I would want would be a reliable clip of at least 10 shots. PVC has the ability to quickly multiply the height of a stack of darts, if they are all in shells. I thought that the PETG tubing which surfaced on the boards a little while ago would be good, because it'd be fairly thin. If I could find something the exact inner diameter of 1/2 inch pvc, but only 1 or 2 millimeters thick, then it would be possible to create a 20 shot clip only 10 or 11 inches tall.
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#12 boltsniper

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Posted 09 March 2003 - 10:37 PM

I know your asking Zero, but since I understand what your idea is a little better i thought i`d throw my 2 cents in. You want a sprung plunger in the gun and use shells to carry the darts. The spring will be reciprocated by an electric motor. OK, the concept is definitely workable as Airsoft has it in operation on all there guns. The only real problem I'm seeing is that it will be hard to build a system to handle the forces of pulling a spring back and then releasing it without bind. The motor would have to be geared down a great deal to attain the torque and force necessary to retract the spring. The other thing I`d worry about is the mechanism that releases the spring when it is fully compressed. I`m not sure how airsoft acheives that. I don`t know much about the workings of their guns. Some kind of ramp at the rear of the chamber to detach the plunger from the retractor might work well. The rest of the gun would be just be of standard homemade gun design. What kind of magazine were you thinking about using? Gravity feed or sprung? I`m gonna try to go for a sprung bottom feed magazine on my latest design. I think if those 2 obstabcles can be overcome the resulting gun would be pretty much a homemade Powerclip but better....maybe. I would probably look into funding an existing high force reciprocating motor system to use. I think building one from scratch that would hold up is probably not gonna be easily done
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#13 Zero Talent

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Posted 10 March 2003 - 12:27 AM

I should have clued in there, my apologies for the wasted time, Merl. I'm not a big fan of what I unaffectionately call "spring boxes," but I'll help as I can. As Bolt loser mentioned, the gear system necessary for the correct torsion in order to pull back a large firing spring would make the system a bit slow, given the slightly larger force you'd need to propel the larger dart projectile (as opposed to Airsoft's plastic BB's, just in general spacial size in addition to mass). I mean, I'd be more a fan of using electrical energy for an electromagnetic firing system, using an induction system (Mike learned stuff over the weekend!), much like that in a spark plug, to power an electromagnetic to fire an electromagnetic air piston forward... But that's not an option. I'll think about that kind of propulsion system, though, see what pops into my head.

As to the clip, yeah, I'd recommend using PTEG or 1/2" CPVC. I know you want to have the shell internally couple with the barrel (Thus the 1/2" PVC Inner Diameter thing), but the easiest thing to do would probably be to use rubber washers to seal the shell before firing.

Here are some precharged air shell designs I've been thinking of. The first one would work, but it's obviously too big, and the other one I can't get working, but is theoretically sound. If you can find the problems with these, please tell me. But this is the best I have right now.

Design 1
Design 2
Design 3

The third design was meant centrally as a hand-held precharged shell, however, so it's just there for flavour. Hope any of this karp helped. If you want to discuss any ideas, my AIM is EpochZerot, and E-mail is.. In my profile. Yeah. Good luck!
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#14 merlinski

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Posted 10 March 2003 - 10:46 AM

The issue with achieving the right amount of torque would be overcome if, as boltsniper said, I gear it down enough.

If it would be possible, I would like to have the motor automatically stop when the spring is taken all the way back, and then start once it is released. I don't want to just start the motor when I pull the trigger, I want the motor to be completely seperated from my actions. When the gun has been fired, the motor pulls the spring back, and stops when it is fully primed.

I think this could be achieved by having the spring piston depress some kind of kill switch when it reaches the fully primed position. When the gun is fired, the the motor will start moving again. This way, when I pull the trigger I don't have to wait for the motor to pull the spring all the way back.

I'm not shooting for 10 rounds in 1 or 2 seconds, like the powerclip. That would need an incredibly reliable clip system, and jam easily. I'm looking more for a semi-automatic system.
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#15 Cadmond

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Posted 10 March 2003 - 08:17 PM

Like zero said, there would have to be a larger spring than in airsoft, therfor requiring more reinforcement. Plus, this is still a plunger gun, and although they can get good ranges, 100-ish feet for some, they're still inferior to air powered, or combustion powered, propulsion. IF we're designing a system with shells and bits, because spring powered guns only had the advantage in ROF, for the most part. That's just in wars, not actually making them, because spring guns are usually less complex than air ones. Except, you're complexifying this one, so they'd probably be the same.

I wonder if complexifying is a word....
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#16 Zero Talent

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Posted 10 March 2003 - 10:40 PM

IF we're designing a system with shells and bits, because spring powered guns only had the advantage in ROF, for the most part. That's just in wars, not actually making them, because spring guns are usually less complex than air ones. Except, you're complexifying this one, so they'd probably be the same.

Well, actually, a spring gun is a bit harder to make, as current designs require an external casing, or some complex homemade part. But hey, I'd like to see a good adaption of Airsoft Technology to Nerf.

Sadly, I don't have any Airsoft guns, so I'm a rookie in this. I assume, for the automated priming, you could just have a copper plate adjacent to whatever you're using to pull the spring back (Cord, rod, gear, whatever), with a direct connection to the power source, and have a simple wire brush of sorts make contact with it when the spring is not primed, and drop off when the spring is compressed to the required point. Of course, this design could experience some problems, such as repeated brush contact with the plate when the gun is primed, burning out the motor from rapid start/stop action, so you'd probably best have some setup at the end to ensure the brush clears the plate by a good margin. If the brush was partially insulated, you could bend the contact plate up into a L shape, so when the brush goes to a certain point, the plate pushes it up off the plate.

So, naturally, your problem here is getting the motor to keep going after the contact is let up. A nice little inductor coil would appear to serve this well. Of course, I have a remedial knowledge of electronics, so please, correct me if I'm wrong.

Perhaps a bit too complex, but I'm assuming that most designs will require a tiny bit of movement once the killswitch is activated, to ensure it doesn't restart immediately. Hope some of this karp helped. Sorry we've hijacked your topic, Boltsniper. Thanks for all your help so far, too.
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