I want to unlock the sub-machine gun. I've beaten the game a bunch of times and I been able to get a ranking that high and I'm rather meticulous so I like to kill EVERYTHING and grab every single item... but somehow that doesn't seem to cut it

You have to finish the game as quickly as possible, so killing all the enemies and picking up all the items pretty much guaranties a lame score. The only items you don' t want to miss are the memos from Joseph (0,1 penalty per memo missed), you have to finish under 2 hours (0,1 penalty per added half hour), you have to finish in hard (0,1 penalty in normal), and you cannot save or continue (0,1 penalty per save).

There is also an enemy kill requirement, but I don' t remember how many are required. It' s not a big number though, must be around 120-150, I avoided almost all the enemies and just killed all the weird mushrooms on my way, that did the trick.

That being said you are allowed 2 penalties, because the maximum score is actually 10,2 stars. I chose to save once and finish under 2 hours 30.

If your goal is only 9 stars you are allowed a few more penalties, but I' m not sure how much more. Finishing under 2h30 only costs you 0,1 penalty, but finishing under 5h will cost you much more than 0,6. All the same, miss too many memos and they' ll cost you much more than 0,1 each.

I always thought "2 hours? That's impossible!"
that's before Beat Subway world in 15 minutes. XD