Shimming is a legit fix. The objective of "on-face" is to have the barrels tight against the breech face when the gun is closed and locked. A few thousandths inch gap between the hook and the pit will cause "off-face." Filling those few thousandth with a shim will solve the problem. First, the shim must be mostly imcompressible, like metal or oiled paper. Second, the shim must have some resistance to wear, like kept clean and lubed. The longivity of the shim depends on the quality of lube maintenance, material used, tightness/correctness of original fit, and the pressure of loads used. An attached shim is more convenient at cleaning time, but an unattached shim is easily put back in place after cleaning.
Shimming a bearing, which the hook and pin are, is not news in the world of mechanics; people have been doing it for many years in more demanding applications than on sporting guns.
