I would be fine with your character making [i]guesses[/i], so long as they were written in the correct way. By the way I read it, you wrote it as if it was all things that had been decided and were in fact happening;
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he had the barrel sticking out of the rocks, how his hand held the butt like a machine gun to keep that rapid fire steady, instead using it to slightly guide the rifle for smooth motion.
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I didn't ever say he had the barrel sticking out of the rocks (even I wouldn't make that mistake and I haven't even thought of joining the armed forces), nor was he holding the barrel to guide the rifle. Those were all things you seem to have decided he is doing, and that, my friend, is what we call powerplaying.