It's not a big deal, but it would've been nice if the controls were explained in game. Especially since they are a little unusual; arrow keys for movement + AS or ZX and the surrounding keys for actions would be more intuitive I feel like. Plus, I think those would've felt a little less awkward to control.
Music is pretty cool. Also I like the sound the checkpoints make, I find it amusing.
The sentient gun thing isn't really conveyed well in the game at all, and kinda caught me by suprise when the text on the end screen started talking about it.
I like the shotgun upgrade; making a dash/double jump explosion/gun-themed is cool I think. I also like the double jump is sort of an attack as well. I would've maybe liked if it could also be used as a weapon though.
Movement as a whole feels awkward though. The lack of acceleration + lack of fixed jump height makes platforming feel very imprecise, which did get to be a bit of problem when the instant-kill lasers started showing up. Also, stopping in place to shoot felt very pace-breaking.
And it's just a little thing, but the flying enemies coming in from off-screen with 0 indicators makes certaom platforming segments kind of annoying.
Overall though, it's a neat game. 👍