trust me, i wouldnt suggest going to an airsoft event sporting a spring gun ^^. you WILL be the "*****" of the event.
.12 bbs are sometimes too small for the barrels of some guns and they simply rool out of you aim it down >_> but they work best for all of my airsoft guns.
there are some really cheap .20 ones i have that sometimes actually get stuck in the barrels of my guns, i have to keep track of which bbs are safe to shoot w/ which gun. due to that i havent bought any more .20 bbs so i cant really give you a definitive answer to those.
i do know this though, if your using a spring gun, dont expect pinpoint accuracy. (unless your target is within 7 feet of you ^^)
i own a few spring guns, and aside from my $200+ tokyo marui spring shotgun NONE of them shoot perfectly straight with any type of bb. (though i do have a 15$ one with a laser sight that shoots on the dot 90% of the time)
get yourself a nice electric or gas gun before you seriously consider going to an airsoft event.
with an electric you dont really have to aim, just make the trail of bbs chase your target down ^^
and with a gas you dont have to worry about readying each shot, + you get 5x more of a kick than a standard spring gun.
they ARE expensive, but the kinda people that go to airsoft wars as opposed to just shooting them off in thier backyard are the ones willing to pay the money for such a gun.
my personal favourite gun is a simple cheap mini electric gun, i paid 45 bucks for it. it shoots 8 bbs per second and has 1000 rd capacity. its only problem is that its gravity fed so i have to keep it upright or it starts skipping shots. but if you get the jump on somebody you can knock em 30-40 times before they can turn around ^^