"...just means carefully & tightly assembled???"
Yeah, basically. Some machining to minimize clearances or tolerances. As example, consider the range for headspace. Blueprinting would result in a chamber with the exact minimum. Another example would be machining of the bolt-face so that it is exactly 90 degrees to the axis of the bore.
Blueprints show dimensions, with some sort of plus/minus tolerances which have been found to allow use of the item. "Blueprinting" means spending the time, effort and money to achieve minimum tolerances. It can be done in manufacturing, but that's why a Rolls Royce costs more than a Ford, or a Purdey costs more than a Remchester.
You're from BATFE? Come right in! I use all your fine products!