a .2g bb @ 400 fps is 1.48J
Why most people use a .2 is because they're inexpensive, and as a game organizer, I can buy a big bag of them and chrono everyone to make sure their guns are shooting 1.48J.
Where this falls apart is just because a gun shoots 1.48J with .2s doesn't mean it will shoot 1.48J with heavier ammo. Phsyics happens and you can in reality shoot 1.48J with a .2 but 1.52J with a .32 or whatever... it can be higher due to the way the air charge is delivered. It's not so much on an AEG but on guns that use expanding gas, like gbbrs and polar stars, you can see a lot more J with heavier ammo.
It's important to remember that your gun should shoot under field limits with the ammo you're going to use. So YOU as a gun owner should be looking after this. If you don't know how, ask your gun tech to chrono your gun with the ammo you shoot and calculate the J by hand.
Why measuring with the ammo you're going to use isn't practical (on a game organization level)
Is that I show up with a bag that says .3s What's to say I didn't empty that bag and load it with .36s which will shoot harder in my setup?
Or I show up with a bag that says .25s that will chrono 350 but in fact they're .2s that chrono 430? It becomes hard to prove what you're shooting to the naysayers that will accuse you of one or the other, and if the chrono ammo is provided by the game staff, it becomes logistically difficult for them to provide a whole slew of ammo weights to do the chrono with.
So chronoing with .2s is the easiest way to get some sort of conformity and logistics simplicity by organizers to make sure everyone is at least close.
If it were me I'd have everyone chrono with .25s to make sure they were all 1.48J At least be in the middle of the weight variances to get a closer reading of what the gun is really outputting... since most people use .25s anyways.
A guy I tech for owns a polar star. He assumed that because his gun shooting 400 with a .2, but when he chronoed with his .32s he was shooting 350fps. A .32 with 1.48J of energy should be closer to 310fps. In this case, he didn't know any better when he started playing with his output settings, but this is a perfect example of people needing to chrono with the weight they will be shooting. His gun is technically not field legal, even though he shoots <400 with a .2.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.
Last edited by lurkingknight; September 12th, 2014 at 15:18..