I heard an interview with Ian Fleming when I was young. He said the point of the original gun was to show that Bond never really needed it. Bond's real weapon was his quick wits, charm, and unfailing intuition, rendering the gun a paper weight to be carried only because it was required. So Bond wanted to carry the most useless hint of a gun he could get away with, which the 0.25 Barretta fit to a tee.
Ian realized that he wasn't getting this point across after being assailed by readers about how awful the .25 Baretta was (Ian's actual point) and that no-one in their right mind would carry a .25 Barretta for field work.
Ian still maintained that Bond really only needed a gun as a last resort, since Bond, himself, was really his best weapon.
Getting a better gun was tacit to Bond saying he wasn't good enough to save the day on is own.