[Floreos]

Does Barrel Length Matter? Yes, but to an extent. Many will think, with our basic understanding of physics as humans, that a longer barrel will "guide" the BB to its target much more consistently. This is not true. The more contact there is on the BB, the more its hop spin will be disrupted, creating inconsistency at long ranges past 50m. This is why 6.01 barrels show good accuracy at short ranges - the BB has no time to curve off-course. However, a BB cannot obtain enough spin from a 100mm barrel to travel 300m, no matter how good the hop up and barrel set up is. So, what length should one aim for? As a rule of thumb in the airsoft tech community, 455mm tends to be the most accurate length - the perfect balance between obtaining hop spin and "over" obtaining hop on the BB. If a barrel is longer, the BB will be in contact with the barrel longer, which can either disrupt the BBs initial backspin or cut off the air flow around the BB which would diminish the hop. 455mm tends to be the length at which the BB backspin "stabilizes" but does not lose its initial direction of backspin. In other words, each shot has the closest consistency in backspin as opposed to longer or shorter barrels. Does this mean a 300mm or 500mm barrel is less accurate? Slightly, but not enough to notice a large difference in game, especially before 70m so don't go putting a 455mm barrel in every one of your guns. 260-500mm will function well for field games and adjust for your play style or weapon platform.
The main point is, a longer barrel does not mean longer range and accuracy. It just means your gun will be longer and, if it is a gas/spring powered gun, it will have slightly higher FPS. Longer barrel lengths will only increase FPS and decrease mobility and accuracy. If you own an AEG, you'll have to buy a new cylinder for that barrel length or bore-up kit because the piston being retracted in the gearbox may "suck" the BB just fired back into the barrel before it exits, creating a jam when firing full-auto on high RoF platforms. The gearbox will also not be able to supply an adequate amount of air for the increased volume of space from a longer barrel.
Avoid barrels longer than 500mm, as they just hurt accuracy and make carrying your gun an annoyance at times. However, there are modifications that can be performed to enhance the effectiveness of any barrel - such as barrel lapping or the LRB mod.

Here is a good article for you.