I heard someone say that the .20g bbs will reduce the fps by a decent amount, can anyone confirm?
You're trying to drive a something with more mass with the same amount of energy. Logically it will have a slower muzzle velocity.
FPS doesn't really mean much, however, and heavier bbs will always have a longer effective range as long as you can get them going 300FPS ish at the muzzle... below that things get a bit slow.
Really .20g bbs are the absolute lightest you should ever use. .12g bbs, as well as being way too light, are never well made. Casting nipples and deformities are terrible for accuracy, bbs should be as slow and regularly shaped and weighted as possible. When I was more into the sport Airsoft Elite bbs were the best bang for your buck, however other companies may have started putting out better stuff now.
But just as an example of bb weights, Tokyo Marui guns with fixed hopups are set for .25g bbs. These are Japanese guns with fairly low FPS (Japanese limit is 1J muzzle energy, Maruis usually shoot around 300FPS with a .2g). .2g bbs ARE light. Don't go lighter.
Also the advertized FPS, especially on a Chinese gun, will generally reflect the velocity you'll get with a brand new gun without a broken in spring with a .12g bb and a 30mph tailwind.
