- 0.20's are general purpose and great for indoors where engagement ranges are shorter.
- 0.25+ are better for outdoors and they are more predictable in the wind and for punching through vegetation. also a bit more of a "punch" than a light BB...less doubt that you got hit by one when shots are taken at long range.
- some hopup setups at certain power levels prefer certain BB weights. For example, I've got one rifle that shoots 0.28's most consistently and another that just seems to shoot better with 0.30's. Not much of a difference really, but I notice that each is more consistent with it's own preference.
- dependant on your mechbox setup. stiff springs use up more power (more resistance). Smaller than 8.4v 1200mah mini batteries are not usefull for more than stock power setups (300fps w/0.20's). 8.4v/1400mah minis are so so for mid power levels (350fps). Large cell batteries have plenty of juice for all day games with lots of shooting. Often the space in the AEG dictates what batteries can be used (i.e. a SIG55x doesn't have much space in it...a folding stock AK usually takes a mini stick type battery...etc...).
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There are lots of FAQ and Stickied posts with tons of information in them. Some are long reads, others are short and sweet. Plough through them and you'll have more info than you'll know what to do with.