For a moment, I couldn't see what you were talking about (the comment about side tubes). How many are there? (Between the low res image and the black color it is kind of difficult to tell.) BTW, I'd consider that more of a ring fin design rather than a tube fin, but that's really just semantics. From what I understand, Craig is right about the length of tube fins and stabilizing efficiency. I think I remember someone mentioning somewhere that about 2X the diameter is about as long as they should be - any longer and you start getting compression effects inside and air doesn't flow through as well.

Something you might want to consider with those side tubes is vented ejection (think of the Estes Trident). Same as a baffle and no (or very little) wadding needed.

Greg