I always see questions on forums like this: “Which barrel length will give me the best accuracy for my gun?”, or “I want to put a longer barrel in my gun. Which length should I get?s The question of barrel length and accuracy are not necessarily related. These questions require a bit of explanation, and there are many factors when talking about barrel length, accuracy, efficiency, etc. It all comes down to volume matching the cylinder volume to the barrel volume.
Here is the formula. I will get into this a bit later.
V = Pi x R2 x H
V = Volume
Pi = 3.1415
R2 = Radius of the cylinder squared
H = The height of the cylinder. Measured from the front of the cylinder to the port (if there is a port in the cylinder)
If you have ever taken a gearbox apart, you will notice that guns have different style cylinders. There are 4 main types of cylinders:
– Full Cylinder (No holes) 455mm-509mm (590 at the most)
-4/5 Cylinder (Holes four-fifths of the way to the back of the cylinder) 407mm to 455mm
-3/4 Cylinder (Holes three-quarters of the way to the back of the cylinder) 247mm or 363mm
-1/2 Cylinder (Holes in the middle of the cylinder) Under 200mm
These cylinders are designed to be used with specific barrel lengths. The numbers above are estimates. For example, if you have a 1/2 ported cylinder, you would not want to try to use a 590mm barrel. This is because the volume of the cylinder (the Height – h) is halved because of the ports in the cylinder, and the cylinder will not have enough air volume to properly push the BB through the barrel. When this happens, the piston has completed its cycle and hits the front of your cylinder head, while the BB is still in the barrel. This causes FPS loss and will actually hinder accuracy/distance. This is especially the case if you are using a heavier BB, because the BB will require more air to propel it through the barrel.
Great, so I have all this information, but how do I put it in to practice? First you want to get the ratio for your gun, which is achieved by taking the volume of the chamber (your cylinder) and divide it by the volume of the barrel (which is basically a long skinny cylinder). When I measured the ratio in my ASG M60, I calculated a ratio of 2.86:1. This means that I had 2.86 times more air volume in the cylinder than in the barrel. Some people would consider this “over volumed”. That M60 had a 363mm barrel with a full cylinder. If you look at the chart above, you will see that a 363mm barrel is not usually paired with a full cylinder.
This seems like a lot of work, do I really need to do this? Not necessarily. If you follow simple guidelines and understand that slapping a 590mm barrel in a CQB M4 gun will not gain you distance or accuracy, you will be fine. Guns tend to be volume matched from the factory (although this is not always the case) and there is a reason why your M4 comes with a ported cylinder. In the case of volume, it would be better to over volume rather than under volume.
How does BB weight factor into this equation? Good question and it is actually quite simple. Let’s say your M4 has a 363mm barrel in it and you want to use a .32g BB. More than likely your gun will not have enough air volume to propel this BB out of the barrel before the piston has completed its cycle to the front of the cylinder. This will reduce the FPS of the gun even more because it requires more “force” (air) to move the BB through the barrel.
How does barrel bore size factor into this equation? Technically, it does factor in, but the difference in the volume you get from a 6.08 when compared to a 6.01 is extremely minimal. Roughly, 98% of the volume will come from the length of the barrel.
So what should I take away from this article? I know this is a lot of information to take in. If you are going to measure your volume ratio, I would shoot for a 1.8:1 to 2:1 ratio to be safe. If you plan on putting a longer barrel in a gun, it is always best to adjust the cylinder also. I have found that it is always better to over volume, as stated before. I hope this helps you understand barrel upgrades in guns. If you are unsure, you can give us a call at the store and we can help you out.