The problem I see that exists with barrel modifications is that the overwhelming majority of these modifications tend to make the barrels entirely too long, and that they place the darts in locations that do not provide for optimum performance. Mind, this theory takes some explaining, so if you don't feel like reading about my physics, take my word and skip down to my conclusion.
First, it is important to establish how Nerf guns work. And I'm talking more in-depth than "plunger pushes air, which pushes dart," because while this is a functional and conceivably accurate description of the process, there's a lot more at play here than simple indirect push-shove of airflow. Now, it may be shocking to some of you, but the darts we love to shoot aren't actually "pushed" out of the barrel, they are actually "pulled." Here's how it works. When the piston (i.e. plunger) shoots forward, the pressure within the chamber (the space between the piston and the dart) rapidly climbs. This difference of pressure creates a vacuum on the other side of the dart. Now, I don't know how many of you have taken high school physics, but it doesn't take a physicist to say that vacuums suck, or rather that in fluids (like air), systems seek equilibrium. The air behind the dart is all trying to be at an equal pressure with the air on the other side. And since the dart can move down the chamber, expanding the volume and thereby decreasing the pressure, the dart tends to very quickly move towards the end of the chamber. As long as the pressure within the chamber is greater than the air pressure outside, the dart will accelerate. In order to maximize the dart's velocity (and thereby its range), the idea is to maximize the amount of time it spends accelerating, which means maximizing the amount of time that the air pressure behind it is greater than the air pressure in front of it.
Now herein lies the difficulty. In an airtight system, any barrel where the dart loses the vacuum in front of it before leaving it is actually incredibly inefficient; meaning any barrel where there is a lack of air pressure pushing on the dart before it leaves the gun and equilibrium can be restored is actually slowing the dart down. This is because once the pressure inside the chamber is equal to the pressure outside, there is no more vacuum for the fluid to flow towards. What happens, then, is as the dart's built up momentum carries it further down the barrel, the pressure in the chamber behind it actually decreases (because the volume is increasing). This means that there is a vacuum behind the dart, and the difference in air pressure is actually pulling back on it, causing it to accelerate negatively ("decelerate"). Practically, this means that the dart's momentum is actually fighting against air pressure if the barrel reaches this length.
To solve this problem, the barrel from where the rear of the dart is placed to the end of the barrel should be equal in volume to the air compression chamber (where the piston and spring are). Here's why. When the trigger is pulled, the compression chamber takes all of its air and rams it into that small area between the chamber's end and the dart. Thus, the volume is the variable that has decreased in the pressure equation, and the easiest way for the system to equalize is to make up that lost volume. As I've said, the dart will continue to accelerate until the pressure is equalized, and if that occurs when the lost volume is made up, then it stands to reason that that would be when the volume from the bottom of the dart (the reverse piston, if you will) and the end of the compression chamber to the end of the barrel is equal to the volume of the compression chamber.
A second difficulty I have seen is that the darts and "breeches" are placed relatively far away from the end of the compression chamber. The problem with this is that the initial volume of the system is larger, and so the change in volume (and thereby the change in pressure) provided by the compression chamber are reduced. Think of it this way: if the compression chamber adds 30ccm' (cubic centimeters) worth of air to the chamber, if there's already 100ccm of air in there, the difference will only be 130ccm:100ccm. If, though, there's only 5ccm of air in the chamber, the difference will be 35ccm:5ccm. For any of you who are good at math, it's fairly easy to point out that 35ccm:5ccm>130ccm:100ccm. For those of you non-math oriented, the 100ccm model only puts 30% more air into the system, where the 5ccm model puts 600% more air. And where there's more air, there's more pressure, and where there's more pressure, the darts will accelerate faster.
So presuming the goal of trying to maximize acceleration, it makes greater sense to place the dart as close to the compression chamber as possible. This minimizes the initial volume, thereby maximizing the pressure difference upon compression and maximizing acceleration. By the same token, it also makes sense that when constructing Stefan darts, a hole should not be burned into the bottom unless it is absolutely necessary. This is because that hole increases the initial volume of the system when it's primed, thereby reducing the pressure difference on firing.
The goal of power modification should be to maximize the acceleration of the darts through the manipulation of the fluid dynamics in the system of the gun. This can be best accomplished by: (1) limiting the barrel--from the bottom of the dart to the end--to match the volume of the compression chamber (a good length for most guns is usually just a little bit longer than the compression chamber), (2) placing the dart's initial spot as close to the end of the compression chamber as possible, and (3) if you're using Stefan darts, to not burn a hole/dent in the bottom of them.
Please also note that this is from a purely theoretical standpoint. As I believe I've said, I have yet to perform a barrel mod to my guns, namely due to a lack of time and resources. I also want to acknowledge that this assumes ideal circumstances, namely an airtight system, which is very rarely the case. It also needs to be said that this does nothing for the accuracy of the gun, which a longer barrel may correct, even if it is at the cost of firepower. I don't expect anyone's guns to be perfect on account of this, but it should help people come close (the closest I've seen is the Fast Action Rifle by Boltsniper, because it exhibits all of these principles).
Edited by Jpec07, 03 October 2009 - 02:55 AM.