Alright I did some more research on this idea over the last few days and found some interesting stuff. The majority of gbb rifles have a fps with .20 gram bbs in the 350-400 range and have a low 300fps range with a .25 gram bb. The .22 calibre lead round ball comes in at 15.43 grains which converts to .99985 grams. This means that there would have to be some tuning and upgrading to the airsoft guns before you could fire the lead ball ammo from it. The heaviest airsoft ammo comes in at .43 grams and I would imagine you would need 600fps at this weight just to get a reasonable fps with the 15.43 grain lead balls. I haven't found a lot of upgrades for the gbb gun I am interested in, a G&G CM16 GBB, but maybe another model by WE or KWA would have more upgradability especially since WE offers co2 magazines for some of their guns. Anyone else have any input or experience with these airsoft guns?