I had the chance to use .25's and .28's and I say for outdoor I would take the .28's any day. They fly truer and increase effective range (but decrease total range). For the velocity hit that you take (heavier objects move slower when momentum is the same) it's still a pretty good tradeoff IMO.
Assuming 400 FPS on .20's (~1.49 Joules) you will get:
360 on .25's (still 1.49 Joules)
340 on .28's (still 1.49 Joules)
Still a decent velocity and the BB still gets to the target in time.
Now lets say you have a 330 FPS setup for one reason or another (lets say you're running a GBB as a primary).
330 on .20's is about 1 Joule:
295 on .25's (still 1 Joule)
280 on .28's (still 1 Joule)
At 280 it's still a decent speed and because of the extended effective range you still come out winning with heavier BB's. That being said however you'll need to lead your shots and predict where your target will go as you have a greater time variance before the BB reaches the target than if you were shooting 295 or even 330.
In CQB situations though I would not recommend heavy BB's .20's are fine for that (since they move slower so you're not being hit by 1J or even 1.13J objects flying at you). As for low powered rifles (ie. 300-330 FPS (exclusive)), .25's or less is probably best (this is my opinion however, you can experiment and see what's right for you).
NOTE: If you use the Arnies Airsoft, airsoft calculator they list effective range however that is incorrect. It should be maximum range not effective range. Effective range is the point where you can predict your shot trajectory before the BB goes off in a weird direction (ie. veering off to the left, right, down, or even up due to hop up). Maximum range is where you can shoot straight and distance the BB goes before it hits the ground and loses all velocity.
Just some things to think about.
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