Please bear with me, since I don't have a good digital camera to help explain my question.  I have a CAR heavy barrel.  

The diameter of the barrel in front of the FSB is of slightly smaller diameter than what is under the FSB.  

So if I take out the pins and move the FSB down to where it would be for a dissy, there is now a slighty gap between the barrel and the inside of the FSB.  Do dissy's have this gap too, or does the inside diameter of that forward FSB compensate for this?

Because I was trying to determine if I can turn my CAR heavy barrel into a dissy by just cutting down the existing FSB and ordering a new 0.75 FSB for the front.  Not sure if a gap there would affect performance, but to me it would look wrong even if it didn't.

there are a couple other diameter FSB out there, I would say measure the barrel and see what the diameter is


or buy a 2 piece and clamp it, or a rail mount sight