It's a nice game. I like the cartoony graphics, the voiceover, the gameplay. It's refreshing to get a gun game that doesn't feel the need to be violent and bloody once in a while. Shock-Dingo's voice and the music go great with the gameplay. I like that you added drop to your weapons, and the guns were relatively detailed without sacrificing the cartoony look. You have a variety of guns, and I feel that a sequel to this should have even more. If so I recommend you add automatic handguns, a few more shotguns and rifles, and perhaps an explosives/launchers section.
However, I feel there's a lot to improve on.
First, I'd like to correct something: Shotgun slugs are a single metal round. What the shotguns are firing here are more like buckshot or birdshot.
Second: I'd like a manual reload button. When I played Defense mode, it sucked having only a few bullets left and unable to reload, then having to reload as three target appear on the screen. If I could reload, it would be a lot more convenient.
Although I haven't completed everything, I noticed that the 10% larger mags perk did not work with all the guns. More specifically, the Walther (assuming this wasn't intentional so as to make the accuracy test more demanding). Similar things have probably happened to others as well, so I suggest double-checking your code if you decide to update again.
Another thing to improve upon: Luck. A lot of things in this game leave so much to chance, especially the accuracy tests for shotguns and some of the skill tests in general. The former is because the buckshot is so random in its spread. I understand this was likely your intention, and I found success if I tried, tried again. The latter, however, was much more out of my control and much more frustrating. If I didn't hit the boxes at the exact time in the exact manner, I could not win at all, nevermind at an expert level. I suggest that you change some of them so that the player is more in control of what happens. This way, it truly tests their skill instead of their luck.
Campaign mode should probably be renamed Mission mode in my opinion, but that's just me. I like that you made Defense mode harder as time goes on so that the player needs to pick better guns as time goes on, and at the same time the low-mid levels can still be done with pistols or SMGs so the player won't be so quick to abandon them.
It was fun to play Efficiency mode. I liked that you had the lives set to be lost based on time. Admittedly, I probably would have based it on hits to the red zone, which no doubt would have made things more frustrating. However, I felt that pistols and shotguns suffered here the most. Pistols have small mags. This means that I will need to reload frequently. Although the reloads on pistols are almost instant, the small hiccups still take precious time away from me. Meanwhile, the shotguns have a wide and completely random spread, coupled with a slow rate of fire. Most of the damage potential of a shotgun was lost here, and high-speed targets could easily dodge the poor aim of the buckshot. Considering that your shotgun also fires very slowly, the low damage output combined with the slow rate of fire makes the shotgun useless in this mode, unless the player wants a challenge.
Offense mode is easily the worst aspect of the game. Although the idea itself is nice, so many things just don't work. Because the bullets drop and your opponent follows your movements, it is impossible to hit them if you plan on firing and then moving upwards to dodge their attacks. Although I like the aspect of how their ferocity reflects the player's weapon, this can be unfair when coupled with how much more health your opponent has than you. The only way to avoid the bullet drop is by using the shotgun, which is still very unreliable with its aim. The best way to play this mode is to use a shotgun, and aim out of my peripheral vision while focusing on my side to dodge the incoming bullets (which doesn't work all the time).
Despite all of this, the game itself is not bad at all. All that needs to happen is fixing a few things here and addressing these faults if you make a sequel. I grade it a 7/10, a decent job.