[McKee]

The gearbox and all airseal components are only ever capable of delivering a maximum force limited by the spring. All parts other than the spring help determine the efficiency of how that spring power is coverted to air pushing power, but the limiting factor is always the spring. So with a force delivered to a bb, the BB's weight will determine how much momentum is imparted to the bb from the air (momentum = velocity x mass). You increase the mass, the velocity (speed/fps) by law must decrease. Therefore you will always drop FPS with a heavier BB, however a BB with a greater mass can better resist the force of wind and will have different flight characteristics under the same hop up conditions you had with a differently weighted bb. This is why hop up must be adjusted to different bb weight.

Don't get wrapped up in joules or FPS.. joules are a measurement of force through a distance, FPS is speed. Joules will never change regardless of BB weight, again thanks to the law of conservation of momentum. FPS will change depending on bb mass. FPS is actually an inferior measurement for this context since in airsoft, we are worried about energy delivered. Needless to say, a .40g bb at 400FPS is going to do a helluva lot more damage than a .20 at 400FPS. If we measured in joules, you would always be able to determine an accurate safety limit regardless of BB used (as long as you knew which one was used).

To clarify lets say you have a gun that shoots a .20g bb at 400fps. If you put .40g bbs in it, you would shoot 283FPS. Your system always delivers 1.48J if this is your setup.

Also, as for distance traveled... air resistance increases exponentially with speed. A higher FPS bb will meet more air resistance. This is why, with hopup, it is possible to shoot a heavier BB farther than a lighter BB despite a lower FPS. OP: You definitely want to use a heavier BB, unless you can find a physics major to help you out, I would recommend just hitting the range and experimenting with a few different weights of BB to see what you can shoot the farthest (just remember the key to success is your hop up setting). Too bad BB bastard doesn't offer a sampler.