OK.......Many above have given you the advice on what shotshell you should be shooting for ducks (& other tips).......3" 1 1/8 3's...1550 fps...The brand in my book makes little difference as long as you stay with the same speed (although, I prefer 1550 Kent's).
First off, there's just some people that can shoot naturally, while others can not.....Most athletics (especially baseball players) I know shoot well.....Why?.....It's eye & hand coordination. Which is very much associated with sports in general.....Imagine a split end running across the field......When you throw the football. You don't throw at the runner. Your brain judges how fast he's running and it tells you how far in front of him you need to throw to hit him as he runs full speed......This is the same concept as hitting a duck as it flies by.....Some folks just get this, some don't.....Rarely will you over lead a duck.....Mostly because it will be farther than you think......Having a range finder will tell you how far your actually shooting.....I rarely shoot farther than 40 yards.
The fit of your gun is very important.....When you close your eyes and throw the gun to your shoulder, you should be looking straight/flat down the barrel.....If it appears your looking uphill down your barrel. Then your gun is shooting high.....So you not only need to lead the bird, but be below it as well.....I can tell you most Remington guns, including 870's shoot flat.....Hence, their popularity to hunters.
I like you, am left eye dominant.....I started my trap shooting life as a lefty, but back in that era, there were few left handed shotguns (I shot o/u's).....I loved my Model 12 trap gun and at about 12, I switched to right handed (I'm naturally right handed). To me, it's never been an issue.....I just close my left eye when firing.....When shooting trap or skeet, where shooting is from a stationary position, I would put the patch/dot over my left eye.
Lastly, your feet and where their placed is really important. I normally move them as I see the angle a bird is coming.....It allows you swing thru the bird.....Unfortunately, in many duck hunting situations your feet are stuck in one position or an other.....Then it's imperative to be able to swing at the hip.....You will normally have to pick a spot in front the bird, instead of swinging & following thru on it in this situation.
Lots of variables and different kinds of shots.....Having a gun that fits will help, shooting the correct duck loads will help, and of course, experience & practice will help.....Good luck
ps.....Here's a little thing I even do to this day.....I take my finger (or even my gun sometimes) and point it in front of birds (sea gulls & such) as they fly by.....You can do this in your yard or blind.....It gives you the concept on that eye & hand coordination needed on when to pull the trigger.
Good luck
