For me, it's all about range and accuracy. That's why I use my M15A4 rifle as my main gun, even though I have shorter AEGs. It will reach out and touch a target farther and more accurately than my shorter barrel guns.
Now, in airsoft, the practical range is usually limited at 125-150 feet for an AEG (maybe a bit more for a finely tuned gun). You have to look at the "extra range" the longer barrels fgive you relative to that short distance.
With .25+ g BBs I can accurately hit a man-size target at up to 150 feet with my M15 with the 20" barrel. After that, accuracy drops significantly. With my JG 416 with the 14.5" barrel, I can hit a man-size relatively accurately out to about 125 and then accuracy drops. That extra 6" barrel only adds about 25 feet, over the 14.5", but that's a 20% increase in effective range, which is pretty significant.
And I agree that bullpup are the shit in airsoft. You get the advantage of a full length rifle barrel in a compact package the size of a carbine.
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