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.
