I managed to complete the game on my first try. Its juicy, the upgrade feels powerful, and the game is very polished. Well done!
What I noticed (and this is an issue I saw with other games with this same concept), the looping between guns doesnt really change how you play the game. I get the the damage multiplier are a things, but they didnt really offer enough incentive nor were they very clear. I am not sure if there exists a quick fix. Maybe make the change more profound, with a big a animation, maybe some changes in the environment. Food for thought I guess.
Nonetheless congrats on your game, it is a truly a good entry!