I completely agree that a chronometer would serve as the ideal method, I've priced them before and have decided they are beyond my nerfing budget (Interesting approach shmmee. Unfortunately, it can be hard to know when this is applicable. I'll offer a more significant problem with using light darts than what has been mentioned. Optimal barrel length is a function of dart mass to a certain extent based on my experience and my computer simulations. I've noted before that when the darts are heavy enough the ideal barrel length is independent of dart mass (as long as this criteria is met, at least). But when the darts are extremely light, it's likely that the pressure in the barrel is going to be different than the pressure in the gas chamber or spring chamber, and this means that your optimal barrel length will likely be significantly different from what it would be if you used "heavy" darts. Is is that bad? I guess it depends, and testing would be the easiest way to see if it is bad.
All the rambling about friction also applies, but based on my simulations, it's not as significant as people think it is. And it's certainly not as significant as the effects from using a low mass dart.
To me, the easiest thing is to have a few barrels of different lengths and a chronometer. I don't really know what anyone's trying to avoid here as barely anyone has even bothered to use a chronometer. It seems to be common knowledge that this sort of testing is bad, but it's not that bad. People seem to spend a lot more time trying to figure out how to avoid using a chrono than it would take to use a chrono. I've done tests like this and it's not that hard or even that annoying. I spent perhaps an hour or two to measure 75ish shots.
I might look into the theory some day later, but I'm worried that it might end up being as much of a boondoggle as the rifled barrel discussion (which I foolishly let myself get sucked into). Had I the time of an uncommitted teenager, I would be more than happy to examine this concept at length, but screw that. I've got kids. I feel lucky to have an undisturbed 3 hours to mod, and defending this concept lays so far at the bottom of my list of nerfing priorities, it's picking pre-chewed gum off of it's face. Thanks for the input everyone,
Doom - as usual - you rock.