The bigger the BB chamber, the more volume there is for the air to fill and therefore the lower the pressure will be, giving you a lower velocity per BB.
You can circumvent this problem in different ways - having a separate hopper as you suggested has already been discussed here (check the following link: http://forums.spudtech.com/topic.asp?AR ... IC_ID=7878
- though I donÃ‚Â´t think has actually been built) and would doubtless work.
Another suggestion which I think is better (and not because itÃ‚Â´s mine or anything
) is to have the BBs actually inside the pressure chamber, meaning you can have as big a chamber as you want with the added benefit of always having maximum pressure pushing them out of the barrel.
http://forums.spudtech.com/topic.asp?AR ... ball,valve