OK, well, you've caught my interest.  There's obviously a story to be told here.

The second question a publisher will ask himself is, Can the writer tell it?  I'm a little concerned that he'll consider these opening sentences too unpolished, and worry that he'd have to spend a lot of time (and therefore money) editing.

"The meth easing further off" is clumsy at best - I take it to mean that the effects of a previous high were receding, but there has to be a crisper way of putting it.  The same with measuring his breaths: may be perfectly accurate, but it's too detailed and acts like a brake this early in the story.

And then, you've set yourself a punctuation conundrum with your last sentence: you've started with a query but can't finish with a question-mark.  Try something more like: "Wondering how much longer he had to pull himself together, his hand went back to his gun."  (Wondered, in this context, works better than thought, which is a bit of a last resort for when nothing else works.)

So, intriguing, but needs tidying up.  Good luck with it.

Flick knew she was in for an entertaining evening when she spotted, on his bookshelf, a well-thumbed copy of The Dummies Guide to Cunnilingus. She kicked off her boots, undid the top button on her uniform jacket, and relaxed on the sofa while she waited for Mack to come down from the attic.

Hey Mark, I'd read on, but on his bookshelf was not really needed and broke up the flow of the sentence. Also top button doesn't create a lot of excitement! :P  And really, that's her relaxing?? ;D

When Mack came back carrying a box containing all the voles in his taxidermy collection, Flick re-fastened her tunic. She also put her knickers and trousers back on; the last thing a girl should do is flash anything fury to a man who enjoyed stuffing small animals.

"Are you leaving?" Mack said

Flick paused and pointed at the cunnilingus guide on the bookshelf. "Why do you have that?"

Mack shrugged. "My nan bought it me for my birthday. I've not started it yet; I'm not keen on Scandinavian Noir. Is it any good."

Flick flicked her hair revealing how she'd got the nickname. "It can drag on a bit, but just when you think it will never end, it reaches a climax."

What a strange woman, Mack thought as Flick left, closing the shed door behind her.