Glad Von Hanson's worked for ya, Bobber.
Usually when I get air in my sausage, it comes from the little pockets within the meat in the cylinder, not between the horn and casing - although it seems like it. I always load the entire length of casing until just a little hangs off the end of the horn (approx an inch), then I crank the stuffer slowly until about the same amount of meat comes out, then I tie it off, air tight. Any air from the cylinder usually takes the path of least resistance - if you have a tight grip around the emerging sausage, the air will usually move up in-between your horn and casing. Not much you can do about it, usually it's manageable. I just try to let it out a little at a time into the sausage, but the only way to avoid it is to take extra care packing the cylinder. But you'll never be able to avoid some air in the mix.