LOL...6.5mm...it'd be like a black powder Brown Bess musket!
I don't think it's practical to do the FPS/BB weight rules with the general populace (or at least enough people to put together a decent sized game).
I purposely kept my last SAW "rough"...I did bump the velocity a bit and just adjusted the hopup so-so. It was so much fun to use since there were so many flyers and I could lay down a pattern of fire but it'd be sort of random luck to actually aim and hit someone with 1-2BBs...but if you suppressed an area it was magic. The other SAW in the squad was basically a high-speed minigun lazer....absolutely deadly...but not the same.