I really wanted to like this game. The concept just sounded really cool. Unfortunately, there are a lot of little mistakes that make this a very annoying experience.
First off, you really messed up the gun and made it really hard to use, which isn't good because this is a shooter. The gun shoots a little to the right of the crosshair, which made it basically impossible to aim correctly. Your bullets are also projectiles, which actually isn't a big deal since the enemies don't move. Just keep in mind that player's gun should always use raycasts, as it feels much more fair, even if the enemies use projectiles.
Speaking of enemies, they are kind of useless. You can't kill them as far as I can tell, just stun them. I think it would work better if it just took one shot to kill enemies, and one shot to die. That might sound weird but I think it would make the game a lot more fast paced. A lot of the time I found myself just waiting in front of an enemy, which was quite boring.
I feel like this mechanic would work better in just a normal shooter game kind of situation, instead of this test chamber type thing. This feels like a tutorial, except the tutorial is the entire game. It felt like you were just hammering the main mechanic into my brain long after I already understood.
I think you were trying to go from a puzzle perspective, but it just didn't work in my opinion. Look at Portal, for example. Portal is great because it takes a simple main mechanic and just does so much with it. Portal, and especially Portal 2 have a ton of other objects and interactable stuff that compliment the portal gun, like cubes you can pick up, jump pads that make you go flying, light bridges, gels, etc etc. You didn't really do anything interesting with the mechanic, because you had nothing to compliment it.
One of the biggest mistakes you can make when designing a puzzle game is thinking you can make the whole game with just the main mechanic and nothing else. Trust me, you can't. Even games like Baba is You, where the directions you can take the main gimmick are almost infinite, have complimentary mechanics.
Anyways, sorry this is so long. I really, really liked the concept, so I wanted to help you as much as possible.