Your first objective is to survive. Always work from cover. Even when you’re just on a patrol always keep an eye out and think “if the shooting started now I’m gonna take cover here or there.”
While it is possible to control your stances with just X ( to toggle crouch) and Z (to toggle prone) There’s a million more stances available to you to get the most out of your cover.
The normal standing stance
You can lower your stance with CTRL-S and heighten it with CTRL-W. This is the first lowered stance. Slightly lower than standing upright.
Another tap of CTRL-S makes you drop to a knee. A great shooting position.
Another tap makes me duck while kneeling. Possibly one of the best stances to be in considering the cover I have. The barrel just makes it over the cover.
Sitting like this works great against targets at elevation with this cover, or when you’re in high grass.
A firing spot has a life expectancy of only a few shots or bursts. Popping out from the same place again is asking to get killed. Be creative with the cover you have.
You can shoulder the rifle on your left shoulder should the cover available dictact you need to pop up from a corner on your left by pressing CTRL-A. Don’t expose your entire body when looking around cover!
You can precisely dictate how much you lean to look around cover if you have a TrackIR.
Like in flight sims, it’s hard to represent both the periphial vision and decent long range vision in a videogame. So Arma has zoom as well.
use zoom-out ( double tap numpad -) to clear urban terrain while looking down your sights like a real rifleman would.
use zoom-in (double tap numpad+) to engage at range. Some sights come with both a reflector sight and a scope, you can use numpad / to toggle between these.
Finally, like Near_Blind said, make sure you find something that is súfficiently bullet resistant as cover.
Wood might stop a 9mm parabellum round
But 6.5mm Grendel ammunition might make it through several layers of it, killing you behind it.
You’ll be hard pressed to find something decent enough against very powerful calibres, like a .50 cal or as is the case with my extreme example, 40mm APFSDS-T.
Not even buildings are guaranteed safespots.
Find the penetration testing mission here!
Hope it was helpful!