Nice game, 8/10
Took me a minute to realize new bullets are fired first, Managing the resources also takes used to, but no game should have a horizontal line as a lerning curve.
Since shells are not able to be combined, they feel wierd to be scavaged the same way as the elements.(personal opinion) They are good the way they are. They just feel off.
Also since you have a wave timer, it is a strange choice to pause the game on lvl-ups.