Yes, harmonics are probably not a major issue for airsoft but stiffness is still very desirable to control general flex, which is more of a concern for airsoft than anything else because of the relatively poor quality of materials and construction. I mean flex as far as handling the weapon goes, especially leaning against cover and barrel mounted bipods, etc. Suffice to say, shorter weapons with fewer bits sticking out tend to be more durable, and will hold their point of impact better when bumped around. This is more in line with accuracy rather than consistency/grouping. Having a one piece barrel very rigidly attached to your receiver definitely helps. (not that the inner can be help very rigidly attached anyways because it has to float in the hopup chamber).
Either way, I doubt shorter barrels are actually any more accurate than longer barrels in airsoft, I just don't think that longer barrels any more accurate either. People seem to put more stock into the idea of longer=better than it ever actually seems to be, especially on the field.
There probably is a shorter limit, but it is likely much shorter than common anyways. The hop up is so much more important than the actual length of the barrel. Look at the VSR10 GSpec - 330mm inner. Look at the TM Mk23 - just 200mm, but still regularly out shooting AEGs. It's because they have such a stable hop up chamber.
Dwell time is the period where the projectile is still traveling down the barrel after having been fired. It's obviously a lot more significant with slow moving projectiles like paintballs and airsoft BBs. The longer the dwell time, the more opportunity for shooter movement to influence the shot, just like lock time.
Regarding paintball, systems exist which provide back spin but I believe the majority do not. It is fairly analogous to airsoft as far as spherical projectile, smooth bore goes though.
I guess what it comes down to is that there are so many MORE effective things you can do to improve accuracy (ammo is probably most important, second is hop up) than worry about barrel length (which as far as upgrades go, it's one of the more expensive ones). While it does make a bit of sense that longer barrels should be more accurate, I feel that this is more of a holdover from looking at actual military firearms, where longer ranges use longer barrels, but even that is more because of larger calibers, minimum fragmentation velocities, and flash/sound suppression than accuracy.
I've yet to see a good explanation for why a longer barrel would be more accurate for airsoft, and I've also yet to see evidence. I'm more inclined to believe that barrel length makes no practical difference at all.
Heck, if I had a lathe on hand to cut and recrown the inners properly, I could probably test it myself... But again, what would be an effective test? How do we factor in hop up? Would it be better to use an AEG as a base system because it is the standard that is in use, but have to deal with variable velocities and porting? Or a regulated gas cannon?
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