Both interpretations are valid and everything in between too.
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I think they should take a cue from D&D and name bonuses for 5e and same named bonuses don't stack but for 4e unless specified they stack.
Yes, I guess that would have been a good idea.
Like:
A sticks with everything.
B sticks not with itself.
C:does not stick with anything.
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Prove there is a rule saying otherwise.
Well, doing it your way SnS would have an armorpenetration of (armor+X)/2 or even armor/2+X. X be the armorpenetration of the rifle you fire it with.
There is a lot of stuff, where it is more than questionable if everything sticks. Healing tests are another thing to consider.
If you go really RAW partial darkness and full darkness would scould tick too.
There is no real rule, but you may have a lot of arguments about this topic anyway...