I got this game with the gold pot on the luck of the Irish sale, and decided to give it a try. And It's OKish, nice time waster.

Anyway, the thing is that I feel like I'm losing weapons somehow. It's happened a few times that a weapon I thought I had on my inventory suddenly disappeared, I thought maybe I pressed a key to drop it or that I may have been imagining things (did I actually buy that stronger shotgun? can't really remember) but on the last level, I actually sold pretty much everything to get the baddass best machinegun available that cuts through zombies like butter. Great stuff.

However, that super expensive awesome machinegun disappeared from my inventory sometime on the level. I can't tell where exactly, much less how, but I'm 100% sure I have bought it and then when I wanted to fall back to it it wasn't there. So I restarted the level, and the same thing happened! Is it some kind of bug? or do weapons deteriorate and disappear on a single mission if you overuse them? Anybody else had this issue?

I know it happened to me near the end, but I thought that may have been because I boosted my cash too high. I had planned on going back and trying without a few million dollars, but if you are having the same problem, I'll assume it's built in. I lost a lot of good armor and guns from that.

The disappearing weapon or armor is a game bug and also appears in Alien Shooter 2.
When buying a high-powered weapon early in the game while having other weapons to cycle through will cause the weapon to disappear. One way I solved this is to not have another weapon to cycle through that weapon type. Weapons are categorized by size or type. By only having one weapon in that category and having no other weapon to cycle through may reduce the chances of it disappearing. It worked for me, but I'm not sure if it is guaranteed.
Also while moving items around in your inventory may cause them to disappear before your eyes, especially armor. I resolved this by not carrying extra armor or just one extra.