A bushing adapter like this one from Aqualabs
http://www.aqualabtechnologies.com/m...IMG_3006_1.jpg
Is meant to stay on the slide so when you pull the slide to clear it comes with it so you clear through an 18.8 joint as per normal, when milking the smallest point of constriction is the hole in the push so effective joint size at that point is no different between a 14.5 or 18.8 slide.
I'm guessing you're either talking about an 18.8/14.5 bushing diffuser which is another kettle of fish or you're kecking the adapter in place on the joint of the tube.