To me it feels this weapon is currently unusable.
The knockback from normal shots makes it extremely dangerous at the depths where you'd use this - imagine being unable to control your movement when fighting Vanaduke or any of the close quarter fights such as arenas or parts of firestorm citadel.
And in a similar way, only more extreme, getting stunned in those situations means that the charged shot is even more of a no-go.
That doesn't feel appropriate for a 5* item.
Not arguing that so much power shouldn't have drawbacks, but at least for this gun, it doesn't currently seem balanced.
I think the inability to move while shooting should not be accompanied by a shortened firing range. On the contrary, I think the Callahan series should have a longer range than other guns.