We all know that crit damage get an extra 2x damage when hitting heads on top of head damage multiplier, it's a pretty common knowledge.
But the gun we're talking about now is The Acceltra:
It has poor accuracy starting from safety range onward.
It also has somewhat slow projectile speed so it'll have a hard time catching those bobbing, moving heads.
And the only range that's suitable for constantly headcritting is below the no-bomb range thus you lose out about 130% extra damage from the explosion in order to hit heads.
If you are still dead on about hitting heads with this gun then, well, go ahead. I won't say anything anymore.
About the impact proc. Nope, it's the worst of all procs since it staggers enemies, making landing headshots even harder. And stagger here means losing balance and wobbling head around trying to regain footing, not completely frozen in place.