Help/Advice with a homebrewed magic railgun!

Hi! Was working on creating a hypothetical modification for a class combo I want to try and ended up with this:

Eldritch Railgun (requirements: force ballista, rune related feat, 5 levels in Artificer, minimum 2000 gp for research and materials (or whatever the DM decides)):

-requires around a month's worth of constant research or around 20 short breaks where u can work on it (with the condition on it that you can't spend your short break doing anything else and have to spend 100 gp minimum during that break - if you spend more than double the amount of money stipulated, then it lessens the amount of breaks required by 1)

-AC 18, HP=5xArtificer Level

-4 elongated sides on the interior of the gun, all w catapult spell engraved on them allowing for a maximum range of 360 feet (720 w spell sniper feat or other things that can modify spell range or range) 

-trigger causes all 4 to be used at once, launching something w 12d8 blunt force damage

-ranged attack roll - not a spell attack roll

-requires:

-2 spell slots per usage to activate the catapult spells

-one weapon action to fire

-one weapon action to prep the ammo itself (that's with the loading already being helped by the artificer infusion - if no feat or trait that helps w loading, then it takes an amount of turns to individually prep all of the shrapnel if using it, otherwise then it's just one)

-used with a replenishing ammo infusion (can prep a bullet in advance, but still requires charging time, and the charge for the runes lasts until the next long rest you take, where you'll have to recharge it again)

-ammo is shrapnel that explodes once the 12d8 blunt damage is dealt

-each piece of shrapnel has a rune on it for a melee magic spell (roll a d10 for the amount of shrapnel inside on hit)

-only 1 spell of your choice can be decided as rune on the shrapnel and once chosen, cannot be rechosen unless you remake the railgun (the spell must be one you have access too and has the touch property or has a range of self)

-the spell can only be a cantrip or 1st level spell and cannot have the damage on it improved by being at a higher level or using a higher level spell slot

-if using a 1st level spell, then you must use a spell slot for each piece of shrapnel and must roll for the amount beforehand to see if you can even do it

-spell choices I'm thinking of:

-magic stone

-shocking grasp

-thunderwave

Does this seem balanced? How could I improve it?

*I should also add the shrapnel part is optional if you're deciding to do one. For my purposes, I want shrapnel bullets.