Jose,
It his been a long time sense I have shimmed a barrel hook. At that time I didn't have the money to send it to a gunsmith.
I bought a blade feeler gauge, used it to measure the distance between the breech face and the barrel face. I then cut the feeler blade, 0.004", and made a shim and placed it in the curve of the hook. I used the gun with the shim in place for awhile. At a later date I took the shim out, softened it with a torch, tinned it with solder, I also tinned the curve of the hook with solder. I then clamped aluminum dowel of the correct radius to hold it in place and soldered it in place, when it cooled I trimmed it with a knife. It worked for me.
Good Luck
