I really liked this game. I loved the mix of 'MS Paint' style art for the menus, level select screen etc and the clean art for the actual gameplay, I got heavy Flash game vibes from back in the day. Speaking of the level select screen, that and the music gave me classic DOOM vibes which I love!
Overall the game was really fun and a cool take on the theme trying out different gun combos. As another commenter mentioned, combat was a little tough from time to time, in particular I think the time it takes to assemble weapons could be shortened (I assume you were going with the idea he's modding the weapons on the fly, thus taking time) but in particular on level 3 getting a weapon at the start I sort of had to wait for quite some time while just getting shot at.
I think this is a really cool concept and you could make it into a bigger game with a bit more polish. Well done, a great entry!