From D&D Wiki
Boomstick (4e Mundane Weapon)
Req: 25 Dex. 15 Con. 15 Str.
Dam: Varies, see special properties
Range: 10 spaces
Special Properties: This blunderbuss variant was created with horrifyingly large wishes of damage output. Due to this, functionality is decreased and this weapon cannot fire if the wielder's attack role does not exceed a natural 9. (Point-Blank bonus is added to this) When 11-14 is rolled, the damage from the weapon is 8d8 x1.25 plus Dex. mod, 15-17 damage is 8d10 x1.5 plus Dex. mod, 18-19 is 8d12 x1.75 and rolling a natural crit, well let's just say this is gonna be so overpowered most dm's will say no, but im gonna say screw you sir! i rolled a crit! awarding the user with 8d20, then roll a d6 to determine which modifier you multiply the score by.
Crit. die roll:
This weapon also has knockback equal to the user's con. modifier and receives a +5 to hit targets that are point blank. (1 Square away) This link provides a general idea of how this weapon would look.