Gough Thomas had written (cannot remember if it was in a book or ST article) of a way to prove action flexing and the need to make room for it by relieving the contact between between barrel and action flats. It involved the use of a small lump of plasteline (play dough in American I think) on the flats and closing the gun. Open and measure the diameter of the flattened plasteline. Now load and fire and remeasure. The plasteline dot grows in diameter after firing, proving that it has been subjected to further deformation due to the flexing of the action bar.
That is a neat experiment but there is no need to go that far. Close any SXS and slide a thin paper, cellophane or feeler gauge between barrels and action flats. That should convince that there is a gap there.
