Ahhh I see your point SRY
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I cannot see any way this could be done within this games limitations.
1. Beerys reduction of firing rate is definetly the best solution as far as the long term fatigue model is concerned.
2. Instant penalties can be added to the deck gun so men get a instant drop to whatever % you wish however this does not work with long term fatigue models.
3. Faster drain rates can be added whilst using the gun but again does not work with long term fatigue models.
4. Weather energy drain can be modded this can work with long term fatigue models buts needs some maths skills and too much testing.
5. Can have it so you need more qualifications.
Most of the above is the only modding options we people know of at the moment so if you are using RUB and want realism then Beery has made the best choice IMHO.