when i open .json-files, i found, that each item have "weight", but each gun have "Weight": 10

all weapon! some`s wrong.... perhaps, was an idea to make different weight-capacity or some similar, but it doesn`t realised.

i guess, if we want to get a realistic game, weight must be realised in full-version.

but it`s seems a great system, because weight must be dependent on: strength, fatigue, injury etc.

and my question to developers: describe how it was to be with weight?

Not sure if you are aware, but current build IS the final version. The team has since disbanded and the studio exists in name only, so chances of continued development are slim to none.

To answer your question from a design point of view, it is likely "weight" was intended to be used when calculating fatigue among others. However, the way production was wrapped up and the game launched, there was little balancing done it seems. Fatigue itself was added very late in the cycle if I recall correctly. Just like skills, such as medical or mechanical, many attributes were not expanded much beyond basic forms. Even "marksmanship" becomes effective at around 85, which seems rather high on a 0 to 100 curve. Basically, the lack of attention and/or resources to expand the scope outside of actual combat, meant many elements exist in a "placeholder" form.