not hard enough
there are many topics about this. Several hundred actually.
very few guns use 8mm rounds. Usually shotguns, some sniper rifles, and some handguns. Few and far between. Everything else is 6mm
GBBs are designed for 0.25g BBs
guns chrony with 0.20g but for the most part, 0.25g out preform the 0.20g.
your not going to find exact numbers on range/weight, since every guns is different, but general rules apply. Heavier is more stable, less effected by wind, punches through the bush better, and I find they have better range.
Personally, you couldnt pay me to use 0.20g in my guns. If I ran out of 0.25g, Id rather use harsh language.
BBs are not cheap. BB bastard has some good prices...and again, buy in bulk, save money.
and this stuff doesnt belong in the general section. place it in the appropriate discussion section.