Well. That is actually a bit of a complicated question because it depends on a few things.
Things like whether the gun is an airsoft rifle or an airsoft pistol.
If it's a rifle, at what velocity does it shoot?
Do you plan on engaging the OPFOR at close range with it all the time, or do you need something more versatile that can handle longer range engagements in an outdoor environment where weather conditions like the wind can come into play?
See what I mean? There's probably a few other factors that I'm forgetting at the moment.
Allow me to provide you with a few recommendations for weights & which platforms I've found they go best with that come straight from my heart, based on the experiences I've had over the years.
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Airsoft BBs for green gas blowback pistols:
Generally, Airsoft gas blowback pistols are going to be relatively low-powered, between 300 fps & 360 fps (measured with a .20g BB). For pistols, I generally run .20g Airsoft BBs if I'm playing indoors and .25g Airsoft BBs if I'm playing outdoors. Most of the time, you're going to be engaging your targets at close range with a pistol, usually within 10 to 20 yards. So I've found that in the whole scheme of things, it's not really necessary to go with a .25g BB. You not only want your BB to get to its intended target as quickly as possible, but you NEED it to because many times at that range, your target knows you're there and is likely going to be firing back at you.
I use the .25g Airsoft BBs in an outdoor environment because the theory is that you're going to gain a little bit more accuracy as the BB travels through the open air. The added weight is said to help cut through some of the wind resistance or something. I don't know. While I do have glasses, I am not physics wizard, or "phyzard."
Indoor, gas blowback rifles & airsoft submachine guns:
For rifles, I'd go with .28g Airsoft BBs. This is assuming your rifle passes chrono to begin with. With gas rifles, you generally want to go with at least a .28g BB at all times, unless you're using something like what I have where you've got a G&P WOC GBBR or a King Arm Colt M4 GBBR with a King Arms low-power nozzle in it, which puts me right around 330fps with a .20g BB. When I run my low-power bolt, I go with the .20g because I need the speed like Maverick (get it?). However, when I've run the KWA LM4 PTR, which, of course, is a gas blowback rifle, I used .28g Airsoft BBs because it fired around 370-380 fps (with a .20g BB) and could handle the extra weight (rather well, I might add).
For submachine guns, indoors or outdoors, I'd go with .25g Airsoft BBs. Real talk.
Outdoor, gas blowback rifles:
When using gas blowback rifles in an outdoor environment, generally the FPS limit is going to be up around 400 fps (with a .20g BB)....you know what? If I tell you an FPS limit, just assume it's being measured with a .20g BB at this point. I'm tired of typing that out. Alright, so field limits at an outdoor airsoft field is generally going to be around 400 FPS. Your engagements are generally going to be at longer distances, so you'll want to find that balance between range & accuracy. A lot of focus by the noobz out there (even me when I first started out) thought that just because a gun had a crap-ton of range meant that it was accurate. However, anyone with brain powerful enough to think will tell you that is simply not the case. Again, I'd go with at least a .28g BB. Some of my associates like to run .30g BBs, but once you exceed .28g Airsoft BBs, supplies become scarce. Not many companies make .30g BBs & even fewer make good ones.