I was super confused about why my gun wasn't shooting and I realized that it was single fire. I think you should have something to show the firing mode of each gun.
Also it can be hard to tell when you purposefully swap and when the game makes you switch. Rather than having the red gun always the the active gun, when you swap the blue gun should come to the front and the red one should get pushed to the back.
After my first play through the enemies stopped spawning in and I had to restart the game to fix it. Sometimes when I fire the grenade launcher(?) the screen doesn't stop shaking for a long time.