Charged Artillery

You conjure an array of 1d4 + your spell ability modifier of small bullet-like missiles that circle the wrist of your dominant hand in magical stasis. While you are concentrating on this spell your affected hand is occupied and you can dismiss this spell on your turn using your bonus action. As an action on each of your turns you can expend up to two of the missiles to attack a target within 60 feet, you can distribute these attacks among targets as you wish. Make a spell attack roll for each missile, on a hit a missile deals 1d12 force damage. 

Each round that remaining missiles are not expended will increase the damage of the remaining missiles by 1 but also increases the chance that the spell backfires due to magical instability. At the end of each of your turns while the spell is still active, make a spell ability check to see if the spell backfires, the DC for this roll is equal to 2 x the amount of rounds your spell has been active. When the spell backfires each creature within 5 feet of you including you must make a constitution saving throw against your spell save DC or take force damage equal to half the amount of extra damage that built on your remaining missiles before the spell backfired.

At Higher Levels. The amount of missiles conjured increases by 1d4 and the damage increase to remaining missiles at the end of your turn increases by 2 for each spell slot level above 2nd. E.g. if you casted this spell at 3rd level you would get 3 extra damage per turn rather than 1.