I faced the same issue. You have to hold LMB longer. It would make a lot more sense if the cannon made a charging sound
I see, thanks for the help. I tried the game again, and I managed to finish it this time so I'll rate it.
The game has nice graphics, the idea is very interesting, you stuck to the theme perfectly with both the mirroring and smoke, the gun behaves how you'd expect it to so that's nice (Good job). I wish there was more to it though, I feel like if your gun can kill the smiling enemy it would be more forgiving as I found myself getting followed by 3 lol
I'd suggest making the first level shorter (as a tutorial), then make more levels that are more difficult if you wanna expand on the game. Either way, great submission!