For the most part a longer barrel works better than a shorter one.

By Grant Woo What's better, a long barrel or a short one? For the most part a longer barrel works better than a shorter one. Why? Well aside from the obvious explanation that a longer barrel will be more accurate many people don't realize it'll also give you more power and velocity. When the air is compressed in the cylinder it creates pressure which leads to force. Pressure, P, is the force acted on an object, F, per unit area, a. P = F / a Force equals mass times acceleration, F = MA. The mass of the BB doesn't change as it travels, so all of the force goes towards acceleration. All objects move in a constant speed (even if that speed is zero) in a single direction unless force is exerted on the object. When force is exerted the speed or velocity will change, meaning there is acceleration. Acceleration is the change in velocity over time. If a car goes 30mph on the street and speeds up to 35mph in 5 seconds, then 40mph in 10 seconds, it is accelerating by 1mph per second. When acceleration stops, that does not mean the object stops moving. In the case of the car, it remains at the final constant speed of 40mph.

P = MA / a

Ok what's that have to do with barrel length? The more time a force is exerted on an object the longer it will be accelerated, and the more it accelerates the faster it will go in the end. Obviously a BB will take longer to travel down a long barrel than a short one, thus it will have air pressure exerted on the BB for a longer amount of time and that means more time to accelerate. This is why you will get more velocity from a long barrel than a short one. Example: a SP110 spring in a M4A1 will give a velocity near 390fps (depending on what other parts are installed), but that same spring in a MP5K will only give 300fps.

HOWEVER, electric powered airsoft guns get their pressure and force only from the air being compressed in the cylinder. It is a specific volume of air that once compressed will want to return to its original volume. Therefore, if the volume of the barrel is larger than the volume of the cylinder than you will get a suction effect in the barrel. Let's say your piston and cylinder will push out 40 cubic centimeters of air and your barrel volume is 60 cubic centimeters. Your piston only has enough air to push the BB two-thirds way down the barrel. Two-thirds way down your BB will reach its maximum velocity and will travel on inertia alone. But in that last third the BB will act like a piston in reverse as it goes down the barrel and cause suction behind it. This suction will want to pull the BB back to its optimal position, two-thirds way down the barrel. Fortunately the BB will have enough inertia to still shoot out, however it will have a lower than optimal velocity because of the suction caused in the barrel will pull it back. In this respect a barrel that is too long will not be good. As an example the HammerMods.Com crew and I experimented with putting a PSG-1 barrel in a M16A2. The A2 was upgraded to shoot 400fps with the A2 length barrel. But with the PSG-1 barrel it shot only slightly over 320fps. Obviously the PSG-1 barrel was too long for the A2 piston and cylinder. Just as a side note the M16A2 has a full, non-ported cylinder, meaning it pushes out the maximum amount of air for version 2 mechboxes.

What does that last statement mean? The cylinders in many of the AEG's have holes in them. This is to port out air. Why do you want air ported out? Well, the concept of force being exerted for a longer time leading to more acceleration is at work here, too. The spring in the mechbox exerts force on the piston making it accelerate. This means that the piston is moving slow in the beginning of the piston stroke and fast at the end. Let's say the barrel has a volume of 20 cubic centimeters. If the cylinder contains 40 cubic centimeters of air you only need to use half of the air inside. To get the optimum performance you want the fastest moving portion of the air, the second half (remember, the air accelerated at the end of the stroke will be moving faster). The first half of the air being compressed is unnecessary because it is moving too slow and will push out the BB before the faster portion of air will even touch the BB. So to get rid of the first half a hole is made in the cylinder to let the air out before it is compressed.

The cylinders in many of the AEG's have holes in them.

My friend and customer experienced this problem when his friend installed an AK-47 full tune up kit in his AK-47B Spetsnaz. This included an AK-47 bore-up cylinder and piston head. He gained a substantial increase in velocity but when he finally checked it on a chronograph it showed lower than expected readings. He told me about what was done to his gun and I advised him to switch his bore-up cylinder to a ported MP5 standard bore cylinder (with matching heads and nozzle). He took my advice and we switched the parts out. This change alone resulted in a 20-25fps increase without changing the spring.

So if you want the most velocity for your parts you should use a long barrel. However, if you do not have a cylinder that matches your barrel you will be losing velocity. So before you put a AK-47 barrel on your MP5, or a SG-1 bore-up cylinder in your G3 SAS, stop and think about what you need to do and how it will affect your performance.