"Steamroller"

"Steamroller" is a comically huge cannon-like gun that fires deadly explosive projectiles.

Damage: 6d10 fire, 3d6 force

When "Steamroller's" energy beam strikes a target, it creates an explosion in a fifteen-by-fifteen-foot area, dealing an additional 4d6 fire damage to all creatures in that area without a successful Dexterity save (DC 15). On a failed save the creature takes half damage.

A to-hit roll is unnecessary if the target is an inanimate object.

 

On a direct hit (the actual target of the attack), the target makes a Strength save (DC 16). On a success, the target is pushed ten feet away. On a failure, the target is pushed fifteen feet away and is knocked Prone.

It’s up to you to decide whether a character has proficiency with a firearm. Characters in most D&D worlds wouldn’t have such proficiency. During their downtime, characters can use the training rules in the Player’s Handbook to acquire proficiency, assuming that they have enough ammunition to keep the weapons working while mastering their use.

Notes: Damage: Fire, Damage: Force, Stunned, long rest actively studying the gun and its components, Range, Two-Handed, Ammunition (Firearms), Reload, Sap