450fps with a metal BB is still not exactly a lethal firearm though, a 9mm pistol for example is typically 1100-1300fps and that's with a seriously heavy bullet (7.5grammes approx for 9mm FMJ). For those who can't be bothered with the maths, that's about 570 joules at the top end. I should caveat that by saying that a 9mm is actually a fairly low energy firearm, the british SA80 (or L85 as you lot insist on calling it!) has a muzzle velocity of nearly 3100fps, with the 4.1g 5.56x45mm NATO bullet it fires. That's 1,767 joules and 5.56 NATO has received some criticism for not having enough stopping power in combat.
Don't get me wrong, it'd hurt like hell and definitely break the skin, but as far as piercing bone and doing actual internal damage? No chance.
In short, NO, your airsoft gun would NOT cut it in the zombie apocalypse, regardless of how big a spring you put in it.