Forgotten Realms: Gond's Boomstick for D&D 5E

In my Forgotten Realms game, I'm creating some magic items to seed my campaign with:

Gond's Boomstick is a double-barreled musket crafted by Diego de Esteban, a chosen of Gond, almost 80 years ago. Gond orders His Church to gift it to a particular Gondswright every few years.  The weapon is known to disappear from its wielder upon their death and return to its resting spot with the Church's vault.

GOND'S BOOMSTICK
Weapon (musket), legendary (requires attunement by a Cleric of Gond)

You gain these benefits while wielding the Boomstick :

• You gain a +2 to attack and damage rolls. Additionally, the musket deals thunder damage. 

• You can fire both barrels as a single attack that deals 5d6 thunder damage, but you must use your reaction to steady yourself against the recoil. You will need to use an action to load both barrels of the Boomstick before you may fire them simultaneously again.

• If you roll a natural 18-20 on the attack roll, the target is knocked prone.