If you're borrowing a gun, you will run through it what the owner tells you to run through it, period.
As for .12's, unless you're using them for area weapons such as airsoft grenades, I wouldn't even try to game them. As stated here, they have no accuracy or distance and you end up frustrated and trying to spray and pray and if its one thing people hate at a milsim game is someone running around with the hammer down full auto the whole time.
Spend the money, buy a quality AEG from a reputable knowledgable dealer. There are no shortcuts to a good AEG.