its because they were hand build for the most part.
Not quite.
It would be more accurate to say they were hand
fit. Early handguns like that were absolutely produced on machines with jigs and set tolerances and such. (Excepting prototypes and preproduction copies and such)
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Hand Made/Built gun is a gun made entirely, or almost entirely by hand, i.e. with hand tools such as files, hand drills, chisels, etc. Some examples would be the Nepalese Gahendra rifles, chinese warlord pistols, Khyber Pass guns, Vietnamese made firearms, etc.
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Hand Fit gun is a gun that is made mostly, or entirely with machines, but has some, often though not always key, parts fit in by hand. The best example of an entirely hand fit gun is the ruby pistols. Theyre all made to the same basic pattern with varying amounts of machine work, but often parts do not interchange and are individual to each gun. Other examples include Luger toggles, upper and lower frames, and triggers, which are all hand fit to each serialized gun. Hanyang 88s are also know for being somewhat hand fit, as certain parts dont drop in and need extra stoning to fit correctly. SMLE bolt heads are also hand fit to each rifle (which is why when you swap bolts/boltheads in those rifles you really need to check headspace and bolthead threads)