Your right, the gunsight doesn't move. The sight was put on a target at a given distance (say, 300 yds) by lifting the tail of the plane and pointing the plane so the sight was on the target. Then the guns were individually adjusted so they each hit the target.
So, in answer to cajunns question- yes, if you set your convergence to 275, your guns are "sighted-in at 275 yds. You should not need to aim high to hit a target at that range (unless you're firing at more than 1x the force of gravity...). You'll shoot low at 600yds...
If you adjust so your convergence is set at 600, your guns are sighted in at 600. You won't shoot low at 600yds...
Easy to test too. Go to the hanger and set your convergence, and use the .target xxx command, while you're in the air.