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Just want to clear something up, I play space marines and have the new Devastator centurions they have grav cannons which are salvo weapons but because of slow and purposeful they shoot as if the didn't move so does that mean salvo has no effect if they moved?

Correct.

Yes they always fire 5 shots at full range.

They cannot fire over-watch though .

Unfortunately...
Even in overwatch they would kick serious ass and deny charges

Question related to centurions and overwatch:
Would giving them Endurance, and therefore Relentless, allow them to fire Overwatch? I'm inclined to say no, and here's why. Both SaP and Relentless are equally "specific" rules. With that in mind, the restriction provided by SaP would override any permissions given by Relentless that would conflict with it.

You're correct. No.
One doesn't cancel out the other. The model would have both SaP and Relentless, and must follow the rules for both.