You'd have to ask someone who actually has a degree for details, but modern guns should be objectively better if we're comparing top of the line guns used by top professional shooters. Discarding any advantage from plastics and other new materials or intention design differences, advances in metal production including use of sensors and computers allow production of materials to tighter specifications. In the real world, metal has a bad habit of changing shape slightly when different temperatures, under different pressure, after repeated load, and so on. By making the metal to tight tolerances, the material can be designed around to reduce the impact of these changes. This would be thrown off by random impurities found more in older metal.

Partially thanks to improvements in metal production, modern machining tools are more precise. If a skilled worker is running the machine, a new machine will produce better parts down to the microscopic (surface faults) level than an older machine, and with CNC and other tools every action can be made in precisely the right location. These may seem minor, but with the force of a bullet even a relatively minor change makes a difference.