SpudFiles

Standard weights? If you have a 4.5mm pellet that weighed, lets say, 0.8g, and a 0.25g 6mm airsoft pellet, which would go further with the same energy? Im just trying to decide which ammo I'll go for.

Pellet would go further. Higher cross-sectional density, less drag.

There's a huge variety of pellet weights available, so I can't give you a definite answer. Here's a chart with the mass of pellets (in grains).
link

If fired from an identical gun the loss of velocity for increased muzzle energy is not proportional to the increase of projectile mass. (generally) - you may double the muzzle energy, but the velocity probably won't drop by half.

The steel BB is heavier and smaller than the airsoft one, therefore it will lose velocity at a slower rate and go further through the air, as well as penetrating deeper through the target medium.

But if you can achieve the same pressure on the airsoft (not likely, but let's say) the larger surface will get a significantly bigger push (19lb at 800psi vs 35lb on a 6mm) and significantly higher muzzle vel. All else being equal, the airsoft doesn't have as tight a fit to the barrels, which may reduce the energy transfer.

A 4.5 mm steel BB will weigh 0.36 grams.

nakarti wrote:But if you can achieve the same pressure on the airsoft the larger surface will get a significantly bigger push and significantly higher muzzle vel.

It'll have a higher muzzle velocity simply by virtue of its lower mass. It'll have a higher muzzle energy by virtue of its higher area.

However, assuming we use the same pressure and barrel length on both, and the BB is doing 200 m/s... anyway, the steel BB would be doing 86.6 m/s.

However, after 20 metres, the steel one would be going faster than the plastic BB. But after only 10 metres, despite the fact that the lead pellet only had 56% of the energy in the first place, it would have more kinetic energy left.

It would however, take 35 metres for the steel BB to "catch up" as it were (less time to target).