(Note: This article is a copy of a discussion by DrPhish and Cam regarding proper weights and FPS for particular BBs. This will be adapted into a full article, so consider this a placeholder.)

A huge thanks to Cam for compiling the information!

A lighter BB means that it can travel faster, and therefore goes further. Right? WRONG!

What is your BB's #1 enemy when it comes to range and accuracy? Wind resistance! A lighter BB has poor wind resistance compared to a heavier BB. In the right weapon, a .25g BB will be more accurate and will have much better range than a .20g BB. A .25g BB has much more wind resistance than a 0.20g BB. As a result, the .20g BB will not travel as far as the .25g BB would. Also, the .25g BB when comparing it to the .20g BB will be more accurate.

If your gun shoots ~350FPS with a 0.20g BB, by all means, feel free to try using 0.25g BB's, you will notice a range increase along with greater accuracy. Plus, your gun will still shoot ~315 FPS.

Gun to BB/FPS relation:
330FPS (with 0.20g) rifles can handle 0.25g.
370FPS (with 0.20g) rifles can handle 0.28g.
400FPS (with 0.20g) rifles can handle 0.30g.
460FPS (with 0.20g) rifles can handle 0.33g.
500FPS (with 0.20g) rifles can handle 0.36g.
550FPS (with 0.20g) rifles can handle 0.43g.
600FPS (with 0.20g) rifles should use 0.43g.

If you have a rifle, and your target is more than 70-80 feet away, you should not be using 0.20g BB's. I think that 0.20g BB's should only be used for CQB battles. Sure, you can try firing a 550FPS rifle with 0.20g BB's, but it wont be any more accurate than a 350FPS rifle with 0.20g BB's. What's the point of shooting fast when your BB's don't hit the target? A newly upgraded gun with more FPS will not give you a considerable boost in range or accuracy unless you couple it with a heavier BB. It's a misconception to think that heavy BBs have less range. Heavier ones have more range, unless you are using 0.43g on stock spring (then it's too heavy). Zero your weapon at around 70-80% of maximum range. That will make the rifle shoot up, and arch down to your target, just like real steel guns.

How do I know how fast my gun will shoot with what BB? To make things simple so we do not have to worry about units, we're going to give the 0.20g BB a weight of 1. On a 0.20g BB level, multiply the following factors to get the velocity conversion:

0.23g - multiply the 0.20g FPS by 0.930
0.25g - multiply the 0.20g FPS by 0.894
0.28g - multiply the 0.20g FPS by 0.845
0.30g - multiply the 0.20g FPS by 0.816
0.36g - multiply the 0.20g FPS by 0.750
0.40g - multiply the 0.20g FPS by 0.710

This will give you an accurate representation on how fast your gun will shoot with what BB.

Every time you give your gun a power upgrade of around 30-50FPS, it is ALWAYS a good idea to knock up the BB weight one level to make the most use out of the extra power. You may notice more range with the power upgrade by itself, but I guarantee you will notice a range, and tighter BB groupings if you upgrade your BB weight, even if it's from 0.20g to 0.23g, you will notice a difference.

Your .25g BB's will not move as fast as the 0.20g BB's when they leave the barrel either, but after about 50 feet, the .25g BB will retain the most energy, therefore it will be going faster than the 0.20g BB.

If you want to improve accuracy, do not use cheap BB's. I highly recommend Airsoft Elite BB's as they are of extreme precision and have a slightly larger outer diameter (5.98mm vs 5.93mm-5.96mm compared to other BB brands), and these BB's are as close to "perfect" as it can get. The larger outer diameter of the BB will help the BB's accuracy as well because it will bounce around less in the inner barrel (which are typically 6.03mm-6.08mm). There are no injection nipples and they are highly polished. There are also other factors that greatly influence accuracy, such as air bubbles, throwing off the BB's center of gravity, as the following figures demonstrate. High grade BB's have less air bubbles when compared to "cheap", low quality BB's.

The following is typical of low quality BB's:



(Extracted from the Bioval BB Lab Research Reports)

Related articles:
http://www.airsoftretreat.com/features/articles/fpsweight/
http://www.airsoftforum.com/board/FPS-Myth-t66494.html
http://www.airsoftforum.com/board/Choice-Ammo-t67615.html
Be sure to check out one of mobaar's educational posts regarding the physicals behind the BB here: here.

Recommended BB's:
Airsoft Elite
TSD
KSC

Hope this helped some of you. :)