Note that this feedback is based on the build(s) available at the end of the jam.
I really like the concept here. Inviting player to be thoughtful about and embrace efficiency for what might be button mashing gameplay in another game.
The three levels included in the game do a good job of introducing and then allowing players to explore the game's mechanics, but there's definitely room to expand this into something bigger if you're keen (and I see from your plan file that you are!).
At the moment, I feel like the game could be more solid with variation in roles between the game's two munitions, which would in turn open up more opportunities for "puzzle design". Perhaps the slug could cause chain reactions that would destroy certain types of targets within a radius, or the explosive could pass over the first target of a certain type - both of these kinds of behaviours would allow each weapon to fulfill roles that the other couldn't, and would provide opportunities for each to hinder or enhance the other's abilities.
I dig the retro aesthetic across the entire game, but the scrolling credits/instructions above and below the main menu is the bit that really set the scene for me on that front.
Thanks for submitting and best of luck with the project! :D