As a fan of both airsoft and air rifle video's I discovered an air soft gun that shoots 900fps with metal 6mm bb's it's called the ASP APM50 co2 Sniper rifle.
Metal bbs are usually made of aluminum and don't weigh more then one gram. Ammo I used for Airsoft back in the day was .20grams (3 grains)-.25grams (3.85 grains) for short to mid range and .45gr(6.94 grains) for long range. As far as accuracy, I would be happy to hit a man sized target at 100 yards.
The guns is closer to 1000fps than it is 900 when put through a crono (something used to measure fps) . If you want to know more I would try using YouTube for help
Just because it shoots a very light projectile very fast does not mean its more powerful than a pellet gun that pushes a heavier projectile at slightly lower speed. In fact it is still much less powerful. If you calculate foot pounds of energy you will find its not even close. I just did a calculation of a 1 grain pellet going 1000 fps it has 2.22 fpe. An 8 grain pellet going 1000 fps has 17.77 fpe. A 18 grain pellet has 40 fpe. Big difference!