I got an Epic machine gun to drop! Good timing, just when I thought my Uncommon one was not strong enough for Hard stages. I feel like you really need at least a Rare autocannon in this game. I have leveled my Uncommon 5.56 up to 4 star and like I said, it's still insufficient.
I find that watching an ad gives me more resources and more quickly than farming snowman on hard difficulty :/
I have a conspiracy theory that the developers of Zombie Gunship Survival are actually hippie indie developers who made the game, with all its weird design decisions, as a prank on gamers who like games about shooting stuff. Then they discovered that the despite all the weirdness, the game can be quite lucrative for them.