Good game overall
At first it's too easy, then it gets pretty much impossible with very steep difficulty curves at the incremented levels(5,10,15,20). Maybe it'd help to put different difficulty settings? Also, I died at level 22 with like 8 hearts. Maybe it would be better if they auto repaired once you hit 0 health, or better yet, maybe it'd be better to just get some sort of upgrade that autorepairs you as you fire. The overheating thing was a little annoying, not because it happened, but more because you couldn't upgrade it to lower the time it happened. Also, I noticed there was an ammobar for the secondary weapons. Please make sure that theres a reload button in a sequel so I don't have to fire at nothing, sacrificing accuracy, to just reload.
Pretty solid overall, but it just needs to be tweaked slightly before it can be great.