Cool concept! I hope you expand on this on later rounds, would be cool to see the customizations affect gameplay.
Maybe some phone guy telling you the types of enemies which are coming, so you can prepare and customize to optimise the gun for those type of enemies.
Would be cool to see the parts change how the gun acts.
I see a lot of potential and I think you have a great foundation for a great game here.
Good job and good luck! :D