I've always seemed to have achieved better effect with a longer barrel as far as accuracy and grouping are concerned, obviously, with a barrel 4 feet long, you need a gun that is about 5'6" long to use it, and that can be the issue.
It has something to do with the rate of fire as well, as you want the first bb out the end before the next one fires as it can mess up the airflow. There will be a way to work it out, but don't ask me.
That would be some calculation, pretty much impossible to do accurately without some kind of proper equipment.
Craig, you could always buy a shed load of barrels and answer your own question Personally I think you'd have to go impractically big for the gun before you started noticing a difference which would annoy you during play.
Hmm i think about a mile would reduce it to a role surely and yea dev imagine useing that for skirmishing youd need a whole group of guys just to shift it in another direction!
When the volume exceeds the optimal size for whatever piston/cylinder you've got.
Sort of related info here -
Those holes in the side of your cylinder are there for a reason. On shorter barreled aegs, this bleeds out unnecessary air so the bb is propelled with optimum pressure when the piston is at its most effective. If there was no hole it, bb's would plop out the end with a significant drop in fps.
Thus, increasing barrel length will usually bring a slight rise in fps up until the 'sweet spot' (relative to your piston vol/stroke) is exceeded, at which point you would start to get a reduction in fps.
Adjusting the cylinder hole's postion can help balance this, as would changing cylinder volume and/or stroke length.