Neat.
The biggest issue for me was the gun not being FULL AUTO. Maybe even having a need to reload every 30 shots could make it more interesting.
It would desperately need some more visual and sound effects as feedback. I know you said you only like programming, I'm pretty much on the same boat, but there is so much you can do with even very simple assets when you throw math and programming into the mix.
It was hard to get a feel of the direction and velocity because of the lack of texturing on the background and the walls.
And btw, in the end allowing the player to freely go outside the game world and not having even small jokes there was straight up mean :(