People who claim long time experience say that enlarged BC gap (bbl-cyl gap) is caused mostly by erosion from black powder acting on the barrel entry and cylinder. Frame stretch is claimed by some to be the cause. I doubt it from measurements I have taken. Also frame stretch would load the top break latch area which does not exist in my experience. Wear in the contact area of the ejector star button to the receiver can cause increased BC gap. Fore and aft play in the cylinder is seen in either/both BC gap and headspace.
Here is factual info from experience using my .44-40 DA Top Break. It fixes BC gap and maintains correct headspace. Tools used were micrometer, feelerguage and dial indicator.
To begin with the BC gap was max at .018" when the fore & aft slop of the cylinder was taken up and headspace was zero with a cartridge loaded. I put in a spacer of .014 between the lower side of the ejector star and the cylinder. That gave a BC gap of .007 and headspace of .003" with the cartridge loaded - pretty much ideal.
The abovementioned spacer reduced the cylinder end play to .003" without cartridge loaded. Take note that movement of the cylinder rearward is controlled by the button in the center of the star interfacing the frame (recoil shield). Further, the action of the spacer vis-a-vis CB gap can also be obtained by lengthening the aforemenationed ejector button by the desired amount. Chicoine in his book "Gunsmithing Old West Guns" reccos drilling-threading and inserting a screw. He mentions that it leaves the headspace increased and in max cases the firing pin may some attention.
With above success I made a washer .015" thick from sheet metal and installed it around the hex rod that pushes the ejector star. It works perfectly mechanically with ammo I have available. I will do a firing test as soon as I can find some 44-40 cowboy. To be continued -----