Question on combat values and fire values

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Thanks for all the help guys.
Another dumb question, how are machine guns useful?
They count as 1 weapons (obviously), so its strength 1 with (let's take) Lethality 40. Assuming wpn status 100% and crew status 100%, it's grand total is 40 Fire Value (not counting any other modifiers). Which gives us casualty min of 0.4% * 6 = 0.024 (if 6 is Low Combat Value) and max casualty of 0.4% * 60 = 0.24 (if 60 is High Combat Value). Double that for effectiveness loss, which is still very low. What am I missing here , did I misinterpret something? Is it only long range + making units get down to the ground?