How to deal with long-range combat?

Hi. I'm new to the series, but I have a pretty good understanding of objectives and teamwork, so I have that part covered. But what I'm still flailing around with is long-range combat. Give me an assault rifle or a shotgun and I can hold my own in tight spaces, but in open areas like ChuChi I'm completely helpless, even as marksman/sniper. My aim is bad, and I don't quite understand bullet drop/adjusting sights, but I feel like there's something else I'm missing.

I always end up either
A: getting shot before I know what happened, or
B: getting shot by someone that I try to shoot at and miss.

How exactly am I supposed to go about these kinds of scenarios? I see other players taking cover and taking occasional popshots, but when I try that I just get killed. Some help would be appreciated.

P.S. I love this game, I die a lot and get mad but other times I feel like an action hero. It's a fun game, just one I don't fully understand.

How to deal with long-range combat?