If the walls of a gun are thin, easing the forcing cones will take off a little strain according to my barrel man. He also recommends the job if cones are very steep, as this will reduce pressure and recoil will be eased a little. I recently had this done to a client's damascus barreled Purdey that was down to 17 thou towards the end of one tube.
If cones are very steep, you may find anything other than 65mm shells (in a 2 1/2" chamber)get ragged at the ends because they are not opening flat -this produces extra pressure and more recoil and requires surgery unless you stick to the 65mm shells. I had this as an issue with a Purdey re-barrelled in the early 1970s and solved the problem.
If any of the above are a concern, it can be a good idea.
However, doing it for its own sake is not somthing I routinely have done. 'If it ain't broke don't fix it' generally works a guiding principle for me.
