Shame to take this off track...it was turning into quite a good discussion.
@ problem P90 guy
- you answered it yourself...you've swapped things around to pin it down to your hopup rubber. I'd say that you either have a bad setup of the rubber/barrel in the hopup unit...or a mis-match between your nozzle and rubber. Have you reverted back to a "known-good" setup to reconfirm nothing else has changed? Have you tried this new hopup rubber with your old barrel/hopup unit? Change one thing at a time...observe...repeat if necessary.
Re. BB Weights and speed
I've never really needed to use weights beyond .30's for my setups. For most of the guns I've worked on I've found that 360-375fps with 0.20's seems to be a really good sweet spot for accuracy/consistency...but my two go-to setups happen to be 399fps and 430fps rifles (chronied with 0.20s...but they prefer 0.28's and 0.30's respectively). While there's plenty of generalized guidelines out there...when you get down to searching out that last bit of accuracy and range it really comes down to testing each factor that affects BB flight, and it all comes after a solid compression system. To me that means hopup rubber, hopup setting, inner barrel, BB weight. Those are the three factors that are most easily and repeatably changed for most setups. And the settings are obviously different for one FPS vs another...or one BB weight vs. another.....when one part changes the others have to adjust in some way.
For example. My CQBR PTW shoots a very consistent 397-399fps. I've modded the hopup rubber/cage. At that power level it prefers 0.28's...difference between BBBastards and Madbulls is negligible but I go with BBB since they're local guys. 20's...25's...just horrible flight paths. I can adjust the hopup until it's acceptable, but it's never stellar. 0.30's...it's acceptable but accuracy/consistent shot placement at range almost doubles with 0.28's. A minute change to be sure...but it's there. I can adjust it for 0.30's to be really close to the performance with 0.28's...but I can't be bothered. Setup as it is...side by side it'll hold it's own or outright smoke most bolt guns out there for range and accuracy if I do my part.
If I swapped a m90 spring into the cylinder it all changes...BB weights (usually 0.20's at that power level for CQB...but 0.25's for some places), hopup settings all have to change. I can still get a very good setup out of it...way more accuracy and range than is needed for CQB. But since it's one of my field guns I just leave it setup for 0.28's.
How easy it is to change parts...how consistently they remain as you've set them...how long they last....those are some of the key points to having a good setup. So far as length of barrel...to quite a bit of a reach I'd say that once you're into MP5/M4 length range it doesn't really matter...and past a certain point and it simply adds additional considerations to your build.