External striking up and polishing, especially in pitted barrels, removes more metal than internal honing, in my experience.

Internal honing on modern machinery is precision controlled. External filing and polishing is not.

It is possible to have in proof shotguns with pristine bores, well within the proof max of 0.789 (18.9mm) yet thinned by external polishing.

The Belgian system that recorded the weight of the barrels, and made reproof obligatory if there was loss of more than 3 per cent weight had its merits.