The easiest way would be to just hold the turret against the seal with your hand. That's what I do with the firefly turret on my SM750, and it has a larger gap then your turret appears to.
The other easy option (that I'm sure I've seen on here but can't remember where) is to put a thin piece of plastic between the shell and the top part of the turret. It pushes the top part out, which pushes the bottom part against the seal.
Another thing to try (although depending on what you're using for the seal it might be a little tougher) would be to make whatever you have forming the seal thicker. This is the only thing that made my lanard shot gun finally work, but it took forever to get everything adjusted just right.
I really like firefly turrets, hope yours turns out well.
Edited by VelveetaAvenger, 14 July 2010 - 06:04 PM.