The M-16 rifle, for instance, has been Standard A in U.S. service since 1965, and its dimensions and tolerances are still being adjusted as problems are identified and cured. Often, a problem is caused by an odd coincidence of dimensions. One part may be at the small end of the tolerance, and another may be at the top end, causing an obscure problem that is corrected with changes to tolerances, not dimensions. The amazing thing about our AR-10 is that they work so well given the small numbers that have been built. We seldom learn of systematic complaints.
Few rifles arrive for diagnosis.
It takes a while to even learn that a general problem exists, let alone fix it. With the design as generally satisfactory as it is, we will continue to have a hard time identifying and defining improvements, but we will continue to try.