Hi there, well I am by no means an expert on this, but the general idea is that on Sup-Air fields (the ones with inflatables) you use the barrel to push into the obstacle, meaning that you are showing as little of your body and equipment to the opposition as possible with only your barrel poking out.
Obviously by having a longer barrel you can push more of the inflatable out of the way.
The flip side is if you want to have a compact marker (if you play at the front of a team) set-up. A 10 or 12" barrel means you have a smaller marker and can therefore hide yourself and tuck in easier behind smaller bunkers.
It just comes down to personal preference really. Most people tend to use a 14" barrel. Some prefer 16, some 12. The length in itself doesn't really effect accuracy or range...
Ben