btrettel's Nerf blog

random thoughts about the ballistics of foam darts

Archive for January, 2011

Banding rubber bladders

Posted by btrettel on January 30, 2011

When a Nerfer wants to increase the pressure of a bladder, they generally turn to “banding,” the layering of rubber bands on the bladder. I came into this hobby from the similar water gun hobby, and I’ve often wondered why this method is so prevalent in Nerf but not water guns.

It’s not that layering rubber bands doesn’t work. I’m sure it works fine. It’s that layering rubber bands is surely far more difficult than the alternatives that will work even better. In water guns people generally layer bike tubes over bladders to increase the pressure. This is not a new idea—it probably originated in 2000 or 2001. (Water gunners generally call this modification “Colossus” as that’s what it’s traditionally called. See SSC for more information.)

When I asked about this on #nerfchat, the only salient point I heard was that kids are more likely to find rubber bands than bike tubes. I’d like to contest that point. Old bike tubes are not difficult to come by. My father had a ton of them that he kept for various odd projects. Many bike shops give them away for free. Consider the number of tubes needed too. One or two 26 inch bike tires should be more than enough to increase the pressure of a bladder substantially. But to get a similar amount of rubber over a bladder with rubber bands would require tens, maybe even hundreds, of rubber bands. How many rubber bands are people likely to have at home?

Additionally, I previously mentioned that banding is more difficult and time consuming than using bike tubes. Bike tubes generally can be rolled up and slid on a bladder quite rapidly unless one has many layers on the bladder (at which point the modifier should probably stop anyway). Banding requires a lot more effort and time, especially if one wants to ensure that the rubber bands are applied evenly over the bladder so that one part does not inflate while the remainder does not.

I hope my point is clear: Banding should be avoided as layering bike tubes is far easier. Why bike tubes are so foreign to Nerfers but well known to water gun people is not completely clear, but I suspect tradition and a lack of creativity from most people in the hobby has a lot to do with it.

Of course, there are other alternatives as well. Someone could layer latex tubing over the bladder or simply use latex tubing as a bladder. Someone could use a homemade hard pressure vessel with a regulator. There are many options.

Posted in Interior ballistics, Misconceptions, Pneumatics | 2 Comments »

Unfinished Nerf gun with semi-auto valve from 2007

Posted by btrettel on January 27, 2011

Back in 2006 I had an idea to improve a pneumatic Nerf gun my brother made. This Nerf gun had a semi-auto valve. At the time I was interested in semi-auto Nerf guns, but I saw the valve as a more daunting task than an advancement system. Like my brother, I started focusing on only the valve as the advancement system could come later.

I eventually started building the Nerf gun, but I ended the project after I found that one seal I used was too small and leaked. I later decided to end the project, but still, I did not see this as a waste of time as I learned a number of things in the process of planning and building (partly, at least) this Nerf gun.

I called the gun NAM, which stood for Nerf Assault Musket. You can view some images of the Nerf gun in construction here:

Problems with my brother’s Nerf gun

  • The valve was difficult to construct. It required the use of properly sized O-rings, and quite a bit of tinkering was necessary to get it to seal. But when it worked, it worked very well.
  • The valve was difficult to service. To replace the seals (which seemed to be necessary on occasion) you had to basically take apart the entire valve. I didn’t intend to make servicing easier, rather, I wanted to make it unnecessary.

My solution

I replaced the O-rings with rubber sheets. This made sealing easier as the sheets did not have to be any precise size to seal. The rubber sheets basically sealed against the flat part of a 2 inch to 1 inch PVC bushing. I made the following animation to help explain this when I would get around to posting it.

The grey is pressurized air. The left end is the barrel. The right end is the air source. Basically, the valve cycles between filling the gas chamber (when the seal is against the left end) and evacuating the chamber (when the seal is against the right end).

I do not remember much more about the project aside from what I have detailed and linked to here. As I said, I considered this project to be a learning experience, and I kept many of the issues I had with this in mind when making FANG1.

Problems with NAM

Around the time I learned that one part would not seal correctly, I started thinking about the design and realized it had a few problems. Not all of what was listed below necessarily was on my mind back in 2007, but I see these problems now:

  • Size – NAM was going to be a very long Nerf gun. Look at some of my photos; I couldn’t even fit the entire layout in one frame!
  • Weight – PVC pipe was used extensively, and PVC pipe is not a lightweight material.
  • Pressure source – If I recall correctly, I intended to fill NAM up with an air compressor. This is not practical for a Nerf war, and I made no plans to use a human powered pump of any variety (though, strictly speaking, this is perfectly possible with the air coupler swapped for a schrader valve).
  • Lack of dart advancement mechanism – NAM was going to have a simple coupler for a barrel. This may have been okay for a test, but I wanted to go further.

Other things that caused me to change my design

  • CaptainSlug’s ABP5K – This completely changed how I approached designing Nerf guns. Previously I limited myself to mostly PVC components as that was the norm. I did not really consider using components with only a structural purpose, even though that seems to be obvious now. You can definitely see CaptainSlug’s influence in FANG1.
  • Reading more about spud guns – Over the next few months I would read a great deal about spud guns and how spud gunners designed them. Until 2008, I was basically ignorant about what a QEV was. I had assumed it would not have been something I would have been interested in even though I was familiar with them going back to perhaps 2005.

All in all, NAM was an interesting learning experience that definitely was not a waste of time. I need to get back into just tinkering like I did here.

Posted in Pneumatics, Unfinished projects | Leave a Comment »

How far in does a pipe fitting thread?

Posted by btrettel on January 17, 2011

When drawing up homemade pneumatic Nerf guns in a CAD program, I used to wonder how deep a fitting would go in. Eventually I learned that technical drawings from McMaster-Carr and many other places offer this information in the listed “thread engagement”. See the drawing from McMaster-Carr below.

Click on the image to view the full sized version.

This information has finally allowed me to start making accurate drawings of my Nerf guns. See the little icon on the top left for an example. (I admit I was a little lazy and I didn’t cover up the threads in the overlap. This actually is somewhat useful, though, so it’s not a problem.)

Posted in Design, Pneumatics | Leave a Comment »

Pneumatic gun model on NerfEngrWiki

Posted by btrettel on January 17, 2011

While what I have posted is far from complete, I have started posting a derivation of a relatively simple model of a pneumatic gun on NerfEngrWiki. Feel free to use the comments section to ask questions, etc., if you have any. I don’t suspect many people will have questions, but I do know a few folks are carefully examining what I have developed.

I also have started working on a page about valves with a focus on modeling the flow through valves for pneumatic gun models. So far I have used this page to derive in detail (something I have not seen before) one of the valve flow equations that I have seen.

More pages will be developed. I’d appreciate any contributions that can be made, too.

Posted in Interior ballistics, NerfEngrWiki | Leave a Comment »

Kinetic energy density on NerfEngrWiki

Posted by btrettel on January 10, 2011

Today I nearly finished a page on kinetic energy density for NerfEngrWiki. KED is one of the more important concepts in the safety of Nerf guns (and that much was admitted by chiefthe, a former Hasbro engineer). I detail the most important topics about kinetic energy density, however, I do not make any suggestions about what critical KED value to design for. I do not think enough data is available to make that decision. However, I intend to limit the most powerful blasters I make to a KED of 20\tfrac{\text{mJ}}{\text{mm}^2}.

Posted in NerfEngrWiki, Terminal ballistics | Leave a Comment »

Older blog posts at

Posted by btrettel on January 9, 2011

Before I would occasionally blog about Nerf projects at my personal website, Now that I have launched a blog specifically for Nerf, that practice will end.

My old Nerf related posts are accessible in the fang2 and fang4 categories at

Posted in Uncategorized | Leave a Comment »

NerfEngrWiki launched

Posted by btrettel on January 7, 2011

Today I am launching NerfEngrWiki, the Nerf engineering Wiki. This Wiki will primarily be used by myself to organize what I know about how to engineer Nerf guns. Others are welcome to help out, though I do not expect anyone to make that effort.

Posted in Uncategorized | Leave a Comment »

Rifling: Helpful, harmful, or ineffective?

Posted by btrettel on January 7, 2011

By making analogies with real guns, some Nerfers have proposed that rifled barrels may be beneficial for Nerf blasters. But is this true? I will examine the two most popular claimed benefits of rifling, that rifling increases range and improves accuracy, and conclude that rifling as implemented thus far has had no significant effect on range or accuracy and it is not likely to have any effect under any circumstances.

First, the reader must realize that these claims are made most often without any backing. The hypothesis that rifling improves accuracy or range is often made based on misunderstandings of what rifling does. Spinning projectiles do not have less drag. Projectiles are spun to improve stability, as I will explain.

Stability of projectiles

A projectile is stable if it flights straight without overturning. This is desirable as the overturning motion reduces accuracy and range.

Rifling is used to improve the stability of a projectile’s flight. But can the stability of a Nerf dart be improved? In general, the answer is no because Nerf darts get their stability from static rather than dynamic characteristics of the dart.

The simplest way to make a stable projectile is to put the center of gravity far in front of the center of pressure. Details as to why this is stable will be later written in the Wiki. Most Nerf darts get their stability in this way; this is why darts are weighted at their nose.

But, most real bullets are made of a single material and they do not have this desirable weight distribution. Spinning the bullet around its longitudinal axis (as rifling does) can stabilize bullets in this case.

So, by simple examination of the mechanisms involved, we can conclude that rifling won’t have any significant effect on darts with the right weight distribution. Those darts are already very aerodynamically stable. There is no reason to rifle as there will not be any real benefit.

Some benefit from rifling seems plausible for very light darts that do not have the right weight distribution. But this is not an argument for rifling necessarily; adding weight to the front is by far the easiest way to stabilize these projectiles. However, this may not seem to be an acceptable choice for some Nerfers. Very lightweight darts may be desirable for safety reasons, however, there are other ways to improve safety of a dart (like reducing the muzzle velocity) that are far simpler than rifling.

Potential disadvantages of rifling

There are many potentially significant disadvantages to rifling that most proponents of the idea are unaware of. I detail the disadvantages that come to mind below.

  • Increased friction – If done poorly, the rifling could increase friction in the barrel and potentially reduce performance as a consequence.
  • Leaks around projectile – If done poorly, the rifling grooves could allow for air to leak around the projectile, reducing performance.
  • Increased complexity of building – Smoothbore barrels are simpler.
  • Less translational KE – To have a spinning dart, some of the energy that would have been put into translational kinetic energy and have contributed to range is instead put into rotational kinetic energy. Rifling is beneficial when this trade-off improves stability such that range or accuracy is improved satisfactorily. However, the reduction in translational KE may not be acceptable in all cases.
  • Reduction of stability – Poorly made darts may not have their weight distributed evenly around the longitudinal axis of the dart. Spinning could destabilize these darts and reduce range and accuracy.

Examining the accuracy claim with data

In 2009, a Nerfer who went by the handle Landru did some tests to see what effect spinning a dart had on accuracy. He used a setup with a spinning barrel. It is believed that this spinning barrel provides a way to control the spinning without making multiple rifled barrels. The test did not address rifling directly, rather, it addressed the question of whether spinning darts could even improve accuracy.

Landru posted some data that he claimed showed that the standard deviation of the locations of darts spun at 2000 RPM was lower than that from no spinning.

However, Landru neglected any sort of statistical analysis. I made a brief post that demonstrated his methods were flawed. I used an f-test to see whether there was any statistically significant difference between the two groups. Assuming a sample size of 20, I found critical f-values of 0.46 and 2.12 for\alpha = 10%. The f-value of was 1.49. As this was between the critical values, the differences were not statistically significant and consequently we can not determine if they were due to the rifling or random chance.

Landru made no follow-up tests.

Examining the range claim with data

Back in perhaps 2003 or 2004, a Nerfer who went by the handle Vassili tested rifled PETG barrels. He found that the average range of rifled PETG was higher than that of smoothbore PETG. Thankfully, Vassili didn’t claim rifling improved range directly. He only offered a tautology: “When it worked, it worked.” But did it work? Can we attribute any of the differences to the rifling and not random chance?

No, we can not. A t-test suggests the two data sets are statistically the same at the \alpha = 5% level. The critical t-value is 2.65. The t-value of the test for the mean is 1.21. As this is within the bounds we would expect at the 5% level of error, we can confidently state that rifling did not increase range in this case.

However, it can be shown that rifling increases the standard deviation of the range with an f-test (data to be added later). This should lead to a decrease in precision due to a decrease in repeatability (each shot is more variable). It also shows that more shots will have lower range with rifling. These two disadvantages are significant.


Based on the implausibility of the explanation for the benefit for rifling and the lack of evidence to suggest that rifling provides any benefit for Nerf darts, I conclude that rifling is ineffective at best and harmful at worst for Nerf.

2014-09-05: Comments disabled due to spam.

Posted in Exterior ballistics, Misconceptions | 19 Comments »

Blog launch

Posted by btrettel on January 6, 2011

I decided I should put my Nerf projects and random thoughts about ballistics on its own blog. Several people in the hobby are interested in reading what I right but not interested in the rants I post on, so this should work out for the best.

Posted in Uncategorized | 2 Comments »