SpudFiles

I've been doing some thinking lately about the shortcomings of my FANG gun and potential solutions. I would greatly appreciate if anyone could read my thoughts and let me know theirs.

The main problem with the FANG is that pushing Nerf darts like it does can be problematic. I used telescoping brass tubes to avoid problems other have had with making parts seal. That worked fairly well, but one of my tubes is slightly dented, making the air cylinder not always return. And even when it the bolt does operate as it should, darts can load incorrectly, still causing problems. Loading darts has been a pain--they tended to fall in at wrong angles before I developed a good way to load. Problems like these were expected because FANG is highly experimental. I feel I can fix the problems to have it operate well, but I shouldn't have to--another approach might be simpler. As you can see, Nerf darts tend to make things a pain--maybe I should stop caring and just shoot something else!

Other problems like the cost and weight are issues too. FANG was designed to use an HPA or CO2 tank. While it's convenient to not pump, carrying that weight probably is just as much work as pumping. The tank and its regulator also were rather expensive. With my computer models and an idea for how much performance is necessary, I can design and efficient gun, so I'm less worried about problems with pumping now. HPA is out and a hand pump is in.

Back to the dart loading cycle. I could keep it the same and work out the kinks, or I could try one of my other ideas. At the moment I'm partial to a rotating barrel assembly not unlike the Eclipse or the RevoSemi. This was one of my original ideas back about a year when I first had the idea to build something like the FANG (not with an air cylinder though). If I use an air cylinder on a cam like the Eclipse, I could rotate barrels and still use the same basic cycle as the FANG. I'm also considering some sort of solid clip, but after some thought I'm not even sure how I would implement that, but if anyone has any ideas I'd like to hear them. Another one of my original ideas used something in between a cam driven system and a clip where the clip was driven by a cam, so that's on the table too, though the main disadvantage to such a system would be returning it to the original position (a rotating assembly doesn't have this problem).

Any thoughts or comments? I'm open to ideas for my second prototype, which I should be starting in the next couple months.

I think it is quite possible to make the FANG system work. When I was working on my semi designs, I also had a lot of trouble getting the ammo to feed in properly, but if you tinker enough you'll get it to work. Its tempting to switch to a different design when one design reveals unexpected problems, but you have to realize that any design at the complexity level of a semi, no matter how simple it looks on paper, will have bugs that need to be fixed.

It'll work well on most projectiles. In fact, I'm excited about designing a similar gun to shoot ball bearings (I wish I had the money though!). It's just that Nerf darts are notoriously problematic and I'd rather avoid problems. PVC Arsenal has had some similar experiences as far as I know.

Let me also reiterate that the FANG gun does work fairly well (much better than I expected) and I'm not giving up on it. It does what I intended it to do, but it gets jammed too often due to a combination of poor construction and unpredictable issues. The construction issues can be fixed and I know I can fix at least most of the issues I didn't anticipate. And it'll be months before I start working on FANG 2--plenty of time to work out the kinks in the first one. In the next week I should get some new brass tubes to replace the bent one and then I'll see what needs to be done. That might be enough to fix all the jamming problems actually because the other problems usually happen when I smack the gun enough to knock the bolt free of the jam.

So yeah, I'm not giving up on the original idea. I'm considering it against the other options.

It might be a good idea to switch to spherical ammo.. seriously unless they are bigger than the barrel they are being fed without any problems...
and that's the simplest solution possible...
plus, it alows you to use a gravity hopper, which is defienatelly the easiest magazine you can imagine

I suppose that problems you encountered are caused not by the action/bolt itself but by either the magazine, ammo or both... obviously the fact that darts are soft doesn't help at all

but all in all I think you should consider either:
*switching to safe spherical ammo (I don't know what to use I suppose you'd have to test different materials and choose one which will have the highest possible mass while still being safe)

*converting it into a more powerful gun that's not intended to be shoot at people... of course again using spherical ammo to make things easier

EDIT
ok I see you are already on the right track

your comment on ballbearings gun is sooo true... I've already fired almost 800 marbles (that's 8 packs, roughly 100 marbles each minus about 4 - 5 ovesized ones in each pack) and it's not that it takes long to fire a whole pack

you already have a high pressure source so converiting FANG into a something really powerful is just a matter of replacing PVC with malleable iron fittings

Spherical ammo would help a lot, yes. Maybe I should have made it clear that I want to shoot Nerf darts because I want to have a gun I can take to Nerf wars. I don't doubt the basic design will work great with spherical ammo.

You can use a gravity fed hopper or magazine with Nerf darts too... it's just that they are so light they can move around and cause bad things.

Edit: Does anyone here have experience welding aluminum air tanks from aluminum tubing? I know how to calculate the stress in thin-walled tubes from my classes (and thick-walled tubes are in my book), so I'm considering using aluminum tubes for an air reservoir. I'm eying this right now: http://www.onlinemetals.com/merchant.cf ... &top_cat=0

My math says the maximum stress will be about 13 ksi at 500 psi, which is a safety factor of about 2.7.

Have you thought about using shells? I know that adds some complexity, but it would eliminate the problems associated with nerf darts bending or jamming. Also shell ejection looks awesome. Here's a design I have used (with a pneumatic ram instead of blow-forward) that was created by another member. It's pretty simple and easy to make:
http://www.spudfiles.com/forums/semi-co ... t8892.html

I have considered shells and I do have experience with them. A Nerf gun I built about 4 years ago used shells. They would eliminate the problems with the Nerf darts, but keeping track of the shells is very annoying. Some sort of chain would help with keeping track of the shells, but I'd still think it'd be a pain.

Rotating barrels as an alternative seem simpler to me, or at least some sort of rotating clip, which could possibly be adapted for a chain. I shouldn't get ahead of myself though.

Shells.

They complicate the hell out of things, but as long as you're using soft cylinderical ammo like nerf darts, they're probably the only way. I've been thinking about how to eject shells for weeks now. I think my latest idea is the most simple. The only moving part is a reverse acting cylinder which ejects the shells.

What I would like to do is take a piece of wood (or PVC stock) and cut and drill it so that its cross section would look like the diagram I've drawn up. Then of course you would have to bolt on blocks of the same size to either end to hold it all together. The back end would be tapped out for a QEV; front end drilled for the barrel.

Shells simply get pushed into place by the magazine spring and remain in that place for firing. After firing, the cylinder extends and pushes them out. The shells themselves seal off the magazine and ejection openings.

I think that PETG would make for good shells. Cheap, easy to cut, and I think I've found a 1/2" sch80 nipple that it fits well in.

To keep track of them easier you could always slap on a shell collector.

The problem of carrying the weight of co2 can be solved by using a remote line and 3.5oz or 88g tank.

If you put the clip on top you can design the gun to simply drop the shells out of the bottom. That's what I would do because it's one less part. Though it might not work.

I'll wait to see what you do with shells before going that way myself. To me, they trade one problem, which might not be a problem with a good design, for more problems. A good approach could change my mind.

So I guess at the moment I'm stuck between keeping the same design and some sort of rotating barrel/clip idea.

The main advantage of a rotating barrel/clip would be that it's simple in some aspects. But it's disadvantage is that it can be expensive with a lot of barrels, it can become heavy unless you use PETG or aluminum, and cutting the path for the cam might be difficult. That's precisely why I stayed away from them before. But I appreciate its simplicity in some aspects... and I might just try it for the heck of it. Using the rapid prototyping equipment at my university to make the path for the cam might be cheating but I'm going to look into it.

The only problem with putting the mag at the top and having the shells drop out the bottom is that you have to come up with a way to keep the shells in place when they need to be, and release only one when it has to.

A rotating barrel idea would be great. But reloading won't necessarily be that easy and neither will construction. But if you can do it, go for it!

That guy on NerfHaven who made the FAR or whatever it is made some cool stuff and he likes sharing them with the community. Problem is, his designs are a bit complicated and hard to build. Most of the nerf community couldn't dream of building complex things. It would be nice someone could come up with a design that's easy and simple enough for anyone to make.

The air cylinder should do what you described with respect to the shells. Though the shell probably will drop prematurely unless there is some other system in place. Just scratch the idea--it was me thinking out loud.

boltsniper's designs are pretty complicated. The FANG was my attempt at something that's reasonably easy to make. It still was too complicated, but I think it was a big step in the right direction. One of my goals was to minimize the amount of machining necessary.

Rotating barrels should be a little complicated to make but easy to load. You'd load them like a revolver. Construction on the other hand will probably be a pain. The main issue is constructing the path for the cam. With rapid prototyping equipment that isn't too big of an issue. I think you can guess what I want to do.

Edit: After examining the options, I've decided to keep the HPA tank in the design and simply use a smaller one. While it is possible to use a thin-walled aluminum tube as a pressure chamber, welding the pieces together would reduce the aluminum's strength by about 80% near the weld. The aluminum is heat treated and reheating it loses that treatment unless it is retreated. Retreating is not something I know how to do or would have the capability to do, so I'll avoid that. Plastic pipe, except for maybe SDR 26 or 21, is too heavy too, so I'll use a smaller HPA tank.

Edit again: SFR 26 or 21 is worth investigating actually...

More thoughts:

FANG 2 will be a branch of thee FANG name with rotating barrels or rotating clips. I'm not giving up on the projectile-pusher approach (see my FANG thread for my developments towards a revision), but I am interested in trying other approaches to see what else works.

The first step in this will be a simpler Nerf gun without an automatic advancement mechanism. About a year ago I made a rotating barrel assembly that is manually advanced and the plan is to adapt a small gun to use that. This should be much simpler and therefore getting good reliability with it will also be much simpler. I can gather a bunch of experimental data with a simpler gun and I can war test it too.

The plan with this intermediate gun is to do a significant amount of testing with it. My computer simulation, BAGS, has approached GGDT in accuracy at subsonic velocities, and given that I don't care about transonic and supersonic projectiles, this is good enough. With BAGS calibrated to accurately model small pneumatic guns I can use it to maximize the efficiency of the gun. I've been running BAGS lately trying to do this and what I've found is very interesting and some things are contradictory to my earlier findings.

Sadly I don't have enough spare money to do most of this at the moment. I get ideas, build them, revise them, and have plans for new things faster than I'm taking in money at this point. All I can do at this point is return some items to McMaster-Carr, wait for Google to pay me (I'm in the AdSense program), and wait for my tax-return. Maybe I'll think of more ideas in the mean time.

One last thing: I emailed Clippard about taking apart their air cylinders and they mailed me a pretty damn nice catalog. They've got a bunch of miniature pneumatic valves I haven't seen before and the flow rates seem good. One valve I'm looking at has a higher flow rate than the equivalent valve from Pneumadyne and is $5 cheaper. I'd suggest checking Clippard out.

Sorry for the triple post, but I came across an important realization that I think everyone interested in making homemade Nerf guns would care about.

Earlier I had discussed my simulation, BAGS, which I wrote specifically to help me optimize Nerf guns (and other pneumatic guns in the future). BAGS is essentially finished so I started running a loop of a hypotherical Nerf gun to see what is efficient. The results were clear: short barrels at high pressures and low volume gas chambers are ideal.

At first I thought "Great... I can't achieve maximum efficiency because my barrel would be too short and then my accuracy would stink." I thought about how longer barrels improve accuracy, and I couldn't understand how they would. After a few minutes of research, I concluded that barrel length and accuracy aren't too related despite what most people believe. Barrels can be less accurate if they are too long because of bending and vibration. Longer barrels can improve accuracy with iron sights because a longer barrel has a longer sight radius, though, sight radius isn't controlled completely by barrel length.

So high pressures with small air chambers and short barrels are the way to go, at least according to my current theory. This will simplify FANG 2. I had planned to emulate the Eclipse because it would be lighter than 8+ rotating barrels. With shorter barrels I don't have to worry about weight. Shorter barrels means a shorter gun too.

Sadly, my theory also suggests piloted valves are inefficient for Nerf because the pilot volume can actually be comparable to the gas chamber volume. I'm designing a simplified and improved HEAR-style valve like the valve used in clide's GBsemi. Eliminating the pilot volume should add about 15% to the energy efficiency and making the valve close at the optimal time could add another 5%, though it likely will only add 1 or 2%. Energy efficiencies of 60% aren't too unreasonable. Without at HEAR-style valve I could manage 35% without difficulty, which is pretty high for a pneumatic gun, but I can do better.

Of course, these efficiency figures assume BAGS is accurate. Most times I've compared BAGS and GGDT were within 2%. At subsonic speeds, when BAGS is off GGDT, BAGS is conservative, which is fine. I am very willing to accept better performance than the model would suggest.

Also, it doesn't seem that I'll have enough time to finish the revision to the original FANG. I bought the parts though so I'll definitely be interested in finishing it eventually. I'll try to finish as much as I can and maybe I can pop back to my parents house for a few days to finish it in the next few weeks.

My first FANG 2 prototype will be complete relatively soon. Today I received some parts made with fused deposition modeling, a rapid prototyping method. Once I finish an essay and take an exam tomorrow I'll begin to finish the remainder of what needs to be done. Most of the remaining work is very simple, however, due to some scheduling difficulties a the gun won't be completely finished for another few weeks.



This gun is highly experimental and the design process will be evolutionary. There eventually will be a second prototype, potentially a third, etc., until I'm completely satisfied and I'll call that prototype the final design.

The main difference between this prototype and the intended final design is that this prototype has a manually rotated barrel assembly. I did this because I want to test if there's any reason to even go semi-automatic. Semi-automatic operation is nice, but introduces reliability and cost problems, so evaluating its merit is worthwhile. There also is the problem of choosing an advancement mechanism... something I'd rather put off until later.

Is FDM worthwhile? It depends. If you're rich, sure. But if you're not rich (like me), FDM either is a convenience or something you use because other methods would not be easily possible. I paid $15 per cubic inch of material... this adds up quickly, but I was allowed to make unusual geometries that I normally would have avoided.

If anyone's interested in more details of the project, go here: http://trettel.org/fang2.html

I also put some updates on my blog: http://trettel.org/blog/cat/fang2

Looking good

About this previous comment:

I thought about how longer barrels improve accuracy, and I couldn't understand how they would. After a few minutes of research, I concluded that barrel length and accuracy aren't too related despite what most people believe. Barrels can be less accurate if they are too long because of bending and vibration. Longer barrels can improve accuracy with iron sights because a longer barrel has a longer sight radius, though, sight radius isn't controlled completely by barrel length

Long barrels are associated with long range accuracy because a long barrel usually means high velocity and this results in a flatter trajectory. The projectile gets to the target quicker and therefore has less time to be affected by gravity, wind and other environmental factors which have to be compensated for.

The other side of the coin however is that with a longer barrel, there's more time in between you pulling the trigger and the projectile leaving the muzzle, so any movement during that time is more likely to have a negative effect on your accuracy.

In short, the ideal in terms of accuracy is to achieve as high a velocity as possible from as short a barrel as possible.