Body armor is different though, because once damaged and penetrated, most body armor becomes useless or at least significantly less protective. And all body armors deteoriate over time, depending on usage and climate. And the fewest of them can actually be mended and fixed up again, but always have to be discarded and replaced.
A punctured steel plate insert cannot be reforged, a shattered ceramic plate cannot be glued back together, a penetrated Kevlar fabric cannot really be stitched back together and regain its full strength.

I rather see this as a dual-value system.
You have one status value for general cleanliness and maintennance, which deteriorates with different speeds depending on gun type, climate and usage.
This one can always be raised from 0% to 100% by simple maintennance and repairs.

The second value represents a permanent wear&tear status, which cannot be fixed by normal means and drops permanently alongside the first value through usage and received damage. And it drops the faster, the lower the first value gets, raising the risk for catastrophical failure alongside. It could also be enhanced by additional status marks, ranging from broken to perfect, according to a certain % treshold. Once its status drops below that treshold, it permanently changes status and according denomination. Expert repairs may be able to improve the overall status to the upper maximum value for the current treshold, but not above, except maybe by using spare parts to replace key components. Maybe even a Fallout New Vegas like repair system would be thinkable, where you can scavenge parts from items of the same type to repair one more effectively?

The key to this system is: the better you take care of your equipment, the slower it will deteoriate through usage.
You may be able to fix a broken or heavily worn gun up well enough to function again, but you will never be able to make it work as good as new. Wear and tear should have a permanent component with limited repairability, in order to make ruggedness, reliability and quality a much more important tactical factor for equipment choice.

Prime example: M-16 vs. AK47. A well maintanied and pristine M-16 may be more ergonomic and accurate than the AK, but the AK will take a lot more punishment and neglect, almost never fail and be much less affected by the environment, whilst the M-16 will greatly suffer from less than optimal conditions and be generally much more maintenance-heavy to field.

We are talking of a scenario and timeframe, where the most available guns will be Vietnam-era stuff and older, with only very few significantly more modern models and higher quality components.

Also, this permanent wear system will force the player to change equipment more frequently, instead of allowing him to acquire it once and use it for the rest of the game, as long as he only has the repair ability. Thus also allowing for another factor to balance out the ingame economy and item progression

Both values influence reliability and accuracy. Because even a relatively well maintained but worn out gun will always be less reliable and accurate than a brand new one, but a badly maintained new gun may become just as unreliable and inaccurate than a well maintained worn gun, until it is properly taken care of.