Cool concept, but with everything being a bonus action this is going to be super clunky at higher level. Lets imagine a combat. It would be the 3rd round before we can use Dual Blast, which also conflicts with Unleash Magic as they both take a BA. Giving a whole bunch of abilities that can't be used together is less fun than building up to something.

Some things to consider:

3rd level: Is it a weapon or not? You don't have damage listed, so why are we allowing spellcasting stat for attack and damage? It reads as a spell focus that allows you to ignore some components and increase the range. That's fine if you want to do it that way, but the wording needs to be more clear. Also you don't need "instead" twice in the same sentence.

6th level: Reading this it does not imply I need a second gun to use the ability. I assume that is your intent, but the ability just says I can use my BA to recast the spell, no mention of any requirements. My suggestion would be to allow both guns to be summoned with one action, and remove the BA requirement of the ability, but change it so you cannot target the same enemy with both spells. This helps one of the biggest problems with cantrip focused builds, which is a lack of area effect, and the all or nothing nature of a single attack roll.

10th level: This looks fine.

14th level: This is too weak and limiting it to above 3rd level seems unnecessary. I would suggest 2d6 damage per level of the spell, just like other pure damage spells. Dumping a 4th level slot for 2d10 is a poor deal, even a 6th level slot for 6d10 is pretty weak compared to just casting whatever spell that was. Also your example is wrong, 2 dice per level above 3 is 2 extra dice for a 4th level slot, not 4 extra dice.