For a setting that I design I deal with REBUILT post apocalyptic weapons.
Sooner or later someone (possibly more than one side) would try to manufacture some weapons. Actually, not just repair existing guns or create some more or less fitting ammo, but provide a few hundred (and in long run thousands) members of some paramilitary with new, standardized guns and ammunition.
Realistically there should be some incomplete blue prints, working or damaged examples, partially complete information about how the weapon should work. Reinventing the wheel is not necessary. The guys hired to do the job are quite bright, some older members even had formal schooling before the apocalypse.
Assuming that there is no need for backward compatibility, to what extend should they realistically copy existing guns (with minor variations caused by initial quality problems)? Or maybe, while reverse engineering deciding that such an old design indeed was quite good, but slightly adjust it to local needs (like different caliber, barrel length, etc.)?
Under normal conditions there is a pressure to keep standard caliber because of logistical needs and compatibility issues. In such a case that would no longer be an issue. Would it mean that at such a moment it should be adjusted? (Based on local conditions, materials accessible at that time and the whim of a warlord... ehm... local military doctrine.) Or maybe with limited resources there would be no choice but to produce (imperfect) clones of existing weapon?
I am especially interested in calibers. Because they are often something that sticks, even after the initial reasons for adoption changed. Should calibers be kept during reverse engineering for simplicity reasons, or would they easily be adjusted based on the needs of the specific situation?