Lately it has become clear to me that a lot of people wanting to tech their rifles, mostly new players, don't really understand the whole thing behind matching cylinder:barrel ratios. This is bad, because with a bad cylinder:barrel ratio you have an inefficient setup. This means you need a more powerful spring for the same fps, you're losing shot-to-shot consistency and if your ratio is too high your rifle will also make significantly more noise.
There are some guides out there or posts that roughly explain the idea behind cylinder:barrel ratios, but this guide is intended to fully explain everything you need to make your rifle optimal, along with the theory behind it. I'll use a different colour for theory you don't need so you can skip it if you wish to.
The reason I'm writing this post is to have some sort of compendium containing everything you should know about this subject. That way I can just link people to this thread in the future rather than explaining the same thing over and over again :hehe:
The cylinder:barrel ratio is a ratio between two volumes, the cylinder volume and the barrel volume. You take all the volume in the cylinder (all the air that's being compressed) and compare it to the volume in the barrel (the area in which it expands behind the bb). If your ratio were 1:1 and your air seal were perfect (no loss of air), then you would end up with zero pressure behind the bb at the moment it leaves the barrel. Intuitively you could say this is what you want, but this is wrong. Your cylinder:barrel ratio should always be higher. Generally something like 2:1 or even higher.
For those of you wondering how to measure the cylinder volume, take the cylinder and measure the diameter (on the inside). Then measure the length (on the inside) from the back side of the cylinder head all the way to the front tip of the piston head with the piston all the way back. If your cylinder has ports (which it shouldn't, we're a sniper forum, everyone here has heavy bb's and ought to be using full cylinders) then measure the length from the cylinder head to the front of the port. The volume is equal to (diameter/2)^2 * length * pi.
The barrel volume is the same idea, but then you take the barrel diameter (6mm) and the barrel length.
So, what's the best
cylinder:barrel ratio? That's a question I sometimes see asked, but the answer is that there isn't. The optimal cylinder:barrel ratio depends on a lot of factors, most notably the bb weight. The optimal cylinder:barrel ratio depends (mostly) on your bb's mass.
[There's an optimal barrel length for every configuration (depending on cylinder volume, bore size, bb weight, bb diameter, spring power, air seal, amount of hop applied and probably a couple of other factors). It is really impossible to calculate exactly what barrel length you should use, just like it is impossible to achieve 100% efficiency. There is a point in the bb's path inside the barrel where the force behind the bb (due to the air pressure) is equal to the total drag force the bb is experiencing (at that point). This is the "maximum efficiency point", and it depends on all those factors. Ideally, for maximum efficiency and quietness, your barrel should stop there. This length very much depends on the bb's mass, because a lighter bb will accelerate faster, lose less pressure, and have covered more distance in the meanwhile. By the time a lighter bb reaches the maximum efficiency point it'll be further down the barrel. For a heavier bb the reverse is true, it'll accelerate slower and cover less distance in more time, which means the maximum efficiency point is earlier than the lighter bb. Of course, the heavy bb loses more pressure because it's inside the barrel longer, but it does so over less distance than the lighter bb so the pressurized air is contained in a smaller volume. According to the universal gas law that means the end pressure is higher. With a same barrel length for both bb's, the light bb will generally have left the barrel before the maximum efficiency point. For the heavy bb this is sometimes also the case, but it's closer, so it'll have used up more of the energy that was in the cylinder.]
Heavy bb's benefit from a shorter barrel compared to light bb's with the same cylinder. In other words, heavy bb's benefit from a high cylinder:barrel ratio.
So, what cylinder:barrel ratio should you use for a given bb weight? Again, depends on multiple factors. Lighter bb's generally stick with 2:1 while heavier bb's can go as far as up to 3:1. .66g super-heavy bb's could go even higher but I have no experience or knowledge on that subject. If you're using .66g bb's I reckon you know what you're doing so you won't need this guide.
Bore size influence
Some people like to install a wider bore barrel. The theory behind this is that the bb rides on a cushion of air, which means less bumping into barrel imperfections, a straighter flight path and more shot-to-shot consistency. The tight- vs widebore barrel debate is still going on, and I'm not going into that here, but if you do install a widebore, this has an effect on your cylinder:barrel ratio.
The barrel volume becomes marginally bigger, but that's not the big deal. The big deal is that a lot of air starts escaping. You will need more air or a shorter barrel, because the bore size affects the efficiency of a cylinder:barrel ratio. A widebore will need a higher cylinder:barrel ratio compared to a tightbore.
[Basically, the cylinder:barrel ratio should be a cylinder-volume:barrel-length:bore-size ratio. The barrel volume can chance in two ways, by changing the length or the diameter. But, if the diameter changes, the length of the bb's path inside the barrel doesn't change. What does change, is that much more air is lost because there's a bigger gap between the bb and the barrel. A 6.23mm barrel compared to a 6.03mm barrel can make quite a difference. This is of course not a linear change, but goes exponentially because a bigger bore results in an even bigger loss of air, just because the laws of physics don't want to make things easy. Therefore it is wrong to think in cylinder:barrel volume, you should be comparing the cylinder volume to the barrel length. For most tightbores (6.01-6.05) the results will be approximately the same, but if you start installing a widebore you will see a serious difference in cylinder:barrel ratio efficiency. Lots of air escapes, which means less pressure behind the bb, which changes the maximum efficiency point (as explained earlier) which in turn changes the cylinder:barrel ratio you should be looking for for a given bb mass.]
I don't have any numbers here, but if you do install anything wider than a 6.05, be sure to get a bigger cylinder:barrel ratio. For example .3 bigger.
Cylinder:barrel ratios and joule creep
You've probably heard about the phenomenon known as "joule creep" sometime. Basically it means that you chrono a gun at a certain amount of muzzle energy (say, 1J), then put different mass (usually heavier) bb's in it, and the muzzle energy suddenly rises! You could for example be shooting 1J with .2g bb's and then 1.5J with .4g bb's.
The cylinder:barrel ratio can be used to explain this phenomenon. Like I said earlier, heavy bb's benefit from a high cylinder:barrel ratio, while the reverse is true for light bb's. When you change your bb weight, you change your optimal cylinder:barrel ratio. Your optimal ratio could come closer to your real cylinder:barrel ratio, which means more efficiency. More energy is put into the bb, resulting in an increased muzzle energy.
A common scenario is where a rifle with a high cylinder:barrel ratio is chrono'ed with .2g bb's. The cylinder barrel ratio could be 3:1 or something, which is not very suitable for .2g bb's. They'll get flung out way before they've used up more energy. When you put .4g bb's in, that 3:1 ratio becomes much more favourable, leading to much more muzzle energy.
[Bore size has an influence here as well. Since heavier bb's are inside the barrel for a longer time, more air will escape. In order to prevent this, you can use a tight bore barrel. A high cylinder:barrel ratio with a tight bore will lead to the most joule creep. A low cylinder:barrel ratio with a wide bore will probably lead to the opposite of joule creep, because most air escapes and the bb actually starts decellerating because it has gotten past its maximum efficiency point.]
Adjusting the ratio
There are four main ways to change your ratio: changing the barrel length, the cylinder (effective) length (porting), changing the piston length or using an airbrake. They all improve efficiency because the correct amount of air is supplied to the barrel and all slightly improve accuracy, but the mechanics used and the effects are different:
-Change barrel length (easiest, preferred), better efficiency so lighter bolt pull.
-Port cylinder, better acceleration and compression so lighter bolt pull.
-Install long piston, short strokes the piston movement, better acceleration and shorter bolt pull but not same pull force.
-Use airbrake, less air to the barrel to get it matched, excess air is used to brake piston and decrease noise.
Changing the barrels length is the most common and preferred method as it is the easiest and provides the lightest bolt pull, while all other methods end up putting energy anywhere other than the bb :tup:
What if I get it wrong?
If your cylinder:barrel ratio is not optimal, don't panic, that's no disaster. It just means you're not running at maximum efficiency. You're losing some shot-to-shot consistency but that's marginal. You will probably need a more powerful spring than you should need.
If your ratio is too high, you will have pressure behind the bb when it leaves the barrel. Of course, this means your setup is not very efficient, but this pressure behind the bb as it leaves the barrel also leads to a distinctive "bang" sound, because all that pressure is released. Thus a high cylinder:barrel ratio leads to fps loss and increased muzzle noise.
If your ratio is too low, you won't have enough pressure behind the bb and it will start decellerating inside the barrel before it can leave. This also leads to instability, which means you'll lose shot-to-shot consistency. Your rifle will be more quiet, but you should avoid this. A high cylinder:barrel ratio will just lead to a lot of noise, but a low cylinder:barrel ratio can mess up your accuracy as well. Both lead to reduced fps.
Of course, the closer you get to your ideal ratio, the less significant these things become.
[A bad cylinder:barrel ratio will always lead to reduced accuracy. With a low ratio, your bb will get unstable, lose the "cushion of air" (which is still very much present in tightbore barrels as well!) and it can bounce into the barrel. This leads to reduced consistency and more fliers. A high ratio means there's still pressure behind the bb when it leaves the barrel. As soon as it leaves, the pressure suddenly goes everywhere. Because this is never consistent, you will lose consistency as the bb is pushed around by, what actually is, a small explosion (also the source of the "bang" sound). With an ideal ratio, the air flow around the bb has the same velocity as the bb itself, so this doesn't happen. These things only matter in high-precision builds though, you can still get decent accuracy without matching cylinder:barrel ratios, but as with everything when it comes to airsoft accuracy: consistency = accuracy.]
Some other factors also come into play, but they're not as important or they're simply not predictable. Ideally, you should have a cylinder-volume:barrel-length:bore-size:bb-mass:spring-power:airseal-efficiency:bb-diameter ratio, but to keep it simple we're not going to make it that complex :hehe:
One thing that you need to keep in mind is that the air seal is important. If you don't have a 100% (or near 100%) seal then the optimal ratio will change. If you don't have a perfect seal then your ratio will need to be higher because more air will escape. You'll also have less joule creep.
I mentioned a "cushion of air" twice. This isn't really the same subject, but I ought to clarify how the bb travels inside the barrel...
The bb is stabilized by a cushion of air, either all around the barrel (the bb is centered in the middle) or below the bb (the bb is pressed to the top of the barrel). Which case is actually happening is a debated subject so I'm not going to go into that here. What is important is that the bb is stabilized by this cushion of air and the absence of this cushion of air is bad for the consistency of your shots. Which is why a barrel shouldn't be too long.
Another important factor is the bb stabilization inside the barrel. It takes some length (say, 100mm, depending on fps) for the bb to stabilize inside the barrel. If the barrel ends before the bb is stabilized it will have bad effects on accuracy. But if your barrel is this short then something weird is going on anyway, so I wouldn't worry about it too much. Just don't go shorter than 200mm, use a little common sense :hehe:
Here you can calculate your cylinder:barrel ratio:
AirsoftTech.dk - Calculator to calculate, Speed, Rate of Fire, Gearsets, etc.
1tonne has written a couple of great guides, and there's a section with ratios (& barrel lengths) for the VSR platform. This contains all the ratios as tested by 1tonne and should be correct ratios (for given bb masses), but of course, with the amount of factors that come into play your optimal ratio could be different.
And here's a rough indication of what (AEG) cylinders you should use with which barrel lengths. Keep in mind that this assumes "normal" barrels, so no tightbores, and standard bb's. If you put heavier bb's in it you will want to change your ratio. These are for approximately 2:1.
FA: Florida Airsoft - Volumetric Ratio Chart
There is a calculator made by HS5 (edited by Boesboes) that can also calculate your cylinder:barrel ratios along with some other things (bore up compensation), although I'm not aware of which calculations it uses so I can't guarantee its accuracy :hehe:
If you want access to this calculator you should send BoesBoes a PM, I deleted the link on his request.
Because everyone and a couple of other people seems to claim this whole post is plagiarism and I've been catching a fair amount of hate for it, I ought to add this. Most of the things I described here were not invented by me, I learned them from other people on this and other forums. This is merely a guide explaining how it works and is not a scientific paper published by me. If anything, I'm only compiling a bunch of posts to make it easier to read and understand for new people. Someone suggested I look up "plagiarism" in the dictionary, and I did. Plagiarism is using someone else's work and pretending it is your own. I am using someone else's work in the broad sense, but then again... isn't everyone who's writing forum posts? I am not, however, pretending it is my own. If anyone disagrees with me, man up and tell me what's wrong so we can work it out.