I thought the theory on that was that a shorter barrel doesn't have as much air to suck in/expel when the piston cycles as a longer barrel does. Don't quote me on this but that's something that seemed to pop out in my mind when I read that.
Before stating my second sentence, let it be known: that is not true of an airsoft gun, and the length of barrel does not affect rate of fire at all.
My second sentence: in days past, when AEGs were almost all built with piston heads that did not have venting holes for unidirectional air flow, this was partially true. Marui designed a piston head with great air compression, but had a problem with back-suction, so to alleviate the problem, they put a hole in the cylinder that left enough air volume to propel the BB out of the barrel (the hole positioned farther back for longer barrels, or no hole at all) but allowing the suction cup effect to only exist within the front area of the cylinder. So, cylinders with no ventilated cylinder (for long barrels) got more back suction, SLIGHTLY decreasing the rate of fire. So, in other words, in stock Marui's this is PARTIALLY true, but most guns now have unidirectional piston heads, so it's not even an issue. Why do they still put holes in cylinders? No explanation for that one, considering there is no negative effects of having a solid bore cylinder with any length of barrel. I don't know if that was too much information, but there you have it, all the same.
-BATMAN
