I've always been of the opinion that 14" is the ideal barrel length. It offers balance and pointability for a back player's setup. 12" just doesn't point on the same plane when brought to the shoulder quickly, rather tending to dance around a bit and not give as smooth a pointing motion and line of reference. 16" can work, but is too tip heavy on some barrel models and can feel disproportionate.
Why all this fuss and wordy description? Simple. A back player has to shoot a lot, switching targets all the time and quickly reacting to people popping up and moving. Thus, aiming must be instinctual and accomplished mainly by how the gun fits and points naturally. You shouldn't have to make your gun go over to where you're looking--when you hold it up, it should already be where you're looking and thinking. If it isn't, adjust tank size, drop forward length, type of gun (You could even get as nit picky as type of milling, as some styles take off more weight in the back, front, or middle, thus changing the balance of the gun), feed neck (high/low/mid), and barrel type until it does.
More often than not I find that a 14" barrel, Drop Zone II or similar length drop forward, and 88 long or 114 bottle make for such a balance.