I do actually aim for a specific spot on a fighter, and have learned to hit it or very close to it on a regular basis.
In any type of shooting, you'll do much better long-term if you aim for a specific spot. It's a method to help refine your concentration.
It does take some practice, and discipline to learn though. But once it's learned it becomes automatic.
In AH, I always aim for the point where the fuselage meets the front edge of the canopy. I don't always hit it of course, but I aim for that same spot, and try to hit that same spot every time. If the plane is turned away so I see the bottom I just aim for that same point on the belly of the plane, imagining my bullets passing through that point and hitting the spot where the front of the canopy meets the fuselage. On bombers I aim specifically for the right wing root when I attack.
In real life, I do the same thing when I shoot. When shooting trap or skeet I focus on the front edge of the clay pigeon. When hunting upland gamebirds or waterfowl I hone-in on the birds beak. When shooting deer I pick a tiny spot to aim at; this might be a specific tuft of hair, etc if the deer is close enough to see that.
One trick I learned as a kid that definitely holds true is "Aim small, miss small. Aim big, miss big."
By that I mean that if you aim at a small spot and refine your concentration to that fine point, you may miss the precise point you're aiming at, but not by much. You aim for the "x", and miss it but still end up in the ten or nine ring. You aim at the tuft of hair over the deer's rib, miss it, but still hit the heart/lung area. You aim at the point where the canopy meets the fuselage and miss it, but hit the pilot, or the engine, or a wing root instead. That's "aiming small, missing small".
When I aim at that tiny spot, and miss by hitting the pilot, I'm thinking to myself "oops, that was a little behind". When I hit the engine I'm thinking "oops, a little too much lead". Mentally, I'm making adjustments so that my next shot is (hopefully) more accurate. I've settled on my aiming point on fighters because I know I have a tendency to fire a fraction of a second late, which often ends up in a pilot shot.
"Aiming big, missing big" is aiming at the deer itself, and missing it. It's aiming at the bird, and missing it. Aiming at the plane, and missing it.
Now, by aiming, I'm saying to look directly at the spot you want to hit, concentrating on it, and firing to hit it. The sight may be on that point, but it's more likely that it will not be. If it is, you'll probably miss. The sight tells you where the bullet will go under certain conditions, and is a valuable tool. However, on a moving target you'll seldom be able to put it where you want the bullets to strike the target. You'll need to learn to allow for trajectory, apparent target motion and angle, and time. You'll need to aim out in front of that moving target to hit it.
The best "trick" for figuring out where to aim in order to hit the target is simply concentration and practice. Focus hard on the point you want to hit, pay some attention to the gunsight in your peripheral vision, and fire when it feels right. If you're aiming right, the bullets will go where you're looking, so "aiming" for a specific spot is really all about concentrating on that spot more than putting the gunsight on it.
Look through the sight, focusing on the point you want to hit. The sight should not really be in focus.
And actually, once you learn what the sight-picture should look like and have your concentration down, you could probably get rid of the sight entirely and still do just fine.
