Unfortunately, there is no magic formula, as there are so many other factors at play. Even if you assume perfect compression (and that's a BIG assumption), the weight of the BBs plus the FPS that your spring is putting out has an enormous effect on BB flight via the relationship between inner barrel length, tightness of bore, and smoothness/consistency of bore. The question is not simply, "how much air do I push through the system," but instead, "how much (cylinder/barrel volume) AND with how much force (spring) for the specific weight of ammo (BB)?"
As far as I know, there has been no comprehensive testing to find this out, and for good reason. There is a long list of variables: cylinder volume, barrel length, barrel bore, barrel quality, spring strength, BB weight, BB quality/consistency. To try and objectively assess their effects on each other without other extant variables such as compression and mechanical efficiency, would be quite difficult simply because of the simple human error element. It would be extremely difficult to find the ideal relationships between these elements even at a set FPS, but then further complicating the issue is of course user preferences with regard to FPS (some like it hot, while others by law have to keep wiithin certain limits). Change the FPS, and everything else changes too.
For instance, even with ideal volume balancing, it has been found that longer, tighter barrels can actually decrease overall efficacy depending on BB weight and spring strength. If you have a weak spring pushing a heavy BB through a long barrel, it will move more slowly than a heavier spring/lighter ammo. This results in increased skipping and bouncing of the BB down the barrel and decreased efficacy.
But then if you were to change one of those factors by swapping out for a heavier spring and/or lighter BBs, that problem could be avoided and you could end up with a wonderfully efficient and accurate system.
I think for your concerns, the best way to go about finding the ideal ratio is to determine what your goals for the project are, and then work toward that. The 1.3 ratio sounds a little too low to me, as it allows no room for error with compression/efficiency. If you're too strict with your ratio, you'll end up having to use lighter ammo. 1.7 should really be the MINIMUM ratio. I would check the ratio on your current setup, and even if it's a little high, just experiement with heavier ammo and see if you can get a fair result that way. Most guns come stock with a perfectly acceptable volume ratio.
What you have to remember is that the single most important factor on the efficacy of your gun is the hopup unit. If you haven't already done all you can to tune, upgrade, and increase the stability and performance of your hop unit, volume balancing won't make any difference at all for you (unless you're using a bore-up kit on an MP5-K or a type 3 cylinder on your sniper rifle).
Edited by buppus, 24 June 2010 - 10:11 AM.