I am in total agreement with Smallbore on this matter, but a couple of caveats for "American" guns. Several makes of US guns used the so called "Taper Chokes" which do not have a paralell section at the muzzle. One of these which has been opened up can be quite readily detected as "Non-Original" so one will need to consider if this will actually be a detriment to it's future value. Also in opening one of these it becomes even more critical that a method is used which insures maintaing alignment with the bore or you may just find it shooting off to a different point of aim.
I realize the gun in the original question here was British, so this would not be applicable to it, but am adding this for others.

Miller/TN
I Didn't Say Everything I Said, Yogi Berra