Lots of ammunition is really old, e.g. 1921 (but really a slight tweak of a 1906 design): https://en.wikipedia.org/wiki/.50_BMG
The key difference here, I think, is that there haven't been radical new improvements in firearms. You can clearly point out where the faults in, say, legacy airplanes lie, but guns (at least in civilian uses) haven't changed as much. Now, granted, for military uses, the modern assault rifle is a far cry from the aforementioned guns.
Just looking at the diagrams in that page, I doubt that measurements were defined with a 0.001 inch accuracy in 1921.
The technology was pretty mature in 1921, there were a few rounds of refinement before that. Machinists definitely talked about "one thou" back then. The same M2s which fire these things have been in production for over 80 years. I'm sure there have been small tweaks, special versions, etc, but basic compatibility has remained.