Pretty cool! The gun felt nice, the projectile visuals were neat as well. Popping the boxes felt good and on my second playthrough I got all the levels in under 20sec. The sliding feels awesome, you could add more "slide down the slope and shoot targets" sections, because they were definitely my favorite part. I like how you got around reloading animations with just spinning the gun.
Very cheeky, I like it :D
On the critical side, the mouse sensitivity is a bit low. I also found a glitch that, if you're going fast enough, the bullets you shoot will hit you - despawning immediately, so if you're moving fast, you sometimes don't shoot. A pretty big problem if you want to encourage speed. On topic of speed - although the timer feels good, I as a player don't really feel encouraged to go fast. Maybe add a small score screen, where depending on your time, you get a bronze/silver/gold medal?