Ballistazooka

The ballistazooka, one the latest and greatest of its kind. Dangerously effective when utilized correctly. 

The ballistazooka is special amongst medieval ranged weapons, reloading it takes an action by default, with the "Crossbow Expert" feat, it instead only takes a bonus action to reload. This weapon also takes a special larger variant of crossbow bolts, simply called "Large Bolts" by most if not all.
On a hit, this weapon deals 2d10 piecing damage, instead of the normal 1d10 damage most heavy crossbows deal. Additionally, when you roll a critical hit with this weapon, the target takes an extra 2d4 damage of the weapon’s type. 

"An unwieldy projectile weapon, dubbed "Ballistazooka" by brigands. The Neoterics of Muraden invented this arcuballista for disputed trials. Those who plead innocence to the Arch Engineer would stand before its narrow sites, praying for the bolt to veer away in an act of divine justice."

Proficiency with a Heavy Crossbow allows you to add your proficiency bonus to the attack roll for any attack you make with it.

Notes: Damage: Additional, Damage: Piercing, Damage, Combat, Ammunition, Heavy, Loading, Range, Two-Handed, Push