The DB Custom barrels are nice, don't get me wrong, but one "lemon" bb or one with a scratch, and you can kiss it goodbye. Unless your personally check each and every bb that you shoot, I wouldn't reccomend, you won't see much of a difference from .02mm however with that extra room if a grain of sand or dirt were to fall in it wouldn't completely ruin the barrel.
You're more than welcome to continue asking advice on these forums as well, I hope that we've helped you in some ways, enjoy your new rifle and be sure to show us some pictures when she's all up and going.
-Mosin