Note that it's a little more complicated than just being a set value - Markerlights for example can affect snap shots / overwatch.
Check out the Snap Shots rule (rulebook pages 32 & 33), the second paragraph tells us that only things which specifically state they affect snap shots can alter the ballistic skill.
So, for example, a Devastator Sergeant cannot boost a heavy weapon forced to snap-fire, despite the Signum being a set value.
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