Got a high score of 8900, just a tiny bit before beating yours. Oh well. The game is very pretty, and I like that old cartoon + war theme a lot. It reminds me of Fallout Nukaworld in a way, and I think you did a very good job on that. The gameplay is simple and easy to understand, and it really makes a good high-score game too!
For a bit of comments, I'd say the game would be more enjoyable (as a high-score game) if the score were shown on the screen at all times. Currently, I need to guess what the score is, and it becomes discouraging when the final score is less than what the players expected it would be. I'd also say there is a strange bug where it's quite easy to "drag" a bullet back onto the screen. It mainly happens for the bullets from the rightmost cannon, and I suspect it has something to do with the mouse being outside the game window. Also, I get that the main point of the game is not letting any bullet touch the ground, but why does it also count as a gameover when a bullet touches another bullet? It caught me off guard the first time I encountered that.
Anyway, this is still a nice game! Keep up the good work!