I have a 20-gauge shotgun with s deep scratch in the barrel just ahead of the chamber. Reaming the forcing cone to a longer taper will clean it up. Do you ream your forcing cones by hand or in a lathe?

by hand

Here is a tool to check the length of a shotgun chamber, chokes, and with a direct read out on the dial indicator. We did lots of shotgun work in 1965 so I made up this tool and it saved much time. We had a full lenth long micrometer, but it took took much time for a quick check. I sold my tool to a guy that made guns shows and wanted to be able to check out shotguns to see whether the shotgun was altered by someone. I made this drawing a long time ago and it needs to be remade to clear up some of the info. Insert the tool into the choke or chamber and zero the dial. As you insert the tool you can read where the forcing cone starts and ends. To get the diameter inside you can use a regular mic to reset the end prongs to the same reading you had on the dial. It would be within one or two thousands for a true mic measurements.

Why worry about it I am sure it works just fine.

Easy enough to do by hand. If you are only going to do one the reamer can be expensive. Also, if the barrel is chrome lined you will need to remove the plating before reaming or get a carbide reamer. If you have a 'smith' do the work make sure that he knows what he is doing. I have seen some terrible work.
C.G.B.

While I can understand how something like the scratch may be of some annoyance every-time you looked down the barrel when cleaning etc, we are talking here about a shotgun where imperfections such as a scratch or even a small dent have no influence whatsoever on the performance of the gun.
Removing chrome plating, if it is chrome lined, does degrade the gun, as would a badly done reaming job C.G.B. has warned of.

I would be inclined to let live and enjoy.