See, the thing about that, is that when you bank the plane the bullets don't do what they're "supposed" to do anymore.
The gun barrels are tilted up when the plane is level, so the bullets start off in an upward arc to compensate for gravity. In addition, the barrel are pointed inward so that the bullets will converge at a given point.
As soon as you bank the plane, that fails to be the case. If you fire while banked 90 degrees left, the barrels aren't pointed upward anymore, so aren't compensating for gravity anymore. A bullet begins falling the instant it leaves the barrel, unless the barrel is tilted upward. If you fired a round from a level barrel, over a level surface, at the same time as you dropped a bullet from the same height as the barrel, they'd both hit the ground at practically the same time. One would be downrange and one would be right next to you.
The result of that is that if you tilt the plane 90 degrees to the left the bullets will hit low, because the barrel isn't tilted to compensate for gravity anymore. Not only that, but the tilt initially put into place to compensate for gravity, is now tilting left instead of up. End result, bullets hit low and left of where the sight says they'll go. In fact, the guns from the right wing will go extra-low, and the guns from the left wing won't be quite as low as expected (because the barrels are pointing inward, which is now a nearly vertical tilt...).
Go further, and roll the plane inverted. Now, the upward tilt to compensate for gravity is actually a downward tilt, opposite of what it "should" be. The effect here is that your bullets will appear to hit high from the perspective of the pilot, which is actually low.
Those effects get worse the farther the sight-line is from the level of the barrel(s). A scope-mounted rifle will do exactly the same thing, but it won't appear as bad (but trust me, you'll still miss). The effect is worse in a plane, because the bullets are aimed to hit a target, at convergence, in your line of sight. Draw a line from your eye, through the sight, to the target. Now realize that in an F4U the guns are roughly 5-6 feet below that line! But at 300yds (or wherever you have you're convergence set), the bullets will cross through that line (or at least meet that line). That's actually a lot of upward tilt! If they come through the line they'll actually hit high immediately beyond the set convergence, before eventually coming back down through the line of sight and hitting low. I never found that to occur with the .50's though.
Coming into this game, I doubted that HTC had modeled the guns/ammunition that accurately, but having done a bit of testing on my own, I'd have to say the modeling is pretty dang close. Those effects occur! They can be tough to detect, but they still occur. They're tough to see sometimes because it's tough to fly on "knife-edge" to fire rounds into the dot-target, and the same is the case for testing inverted. Your nose needs to be tilted upward to fly knife-edge, which means in aleft back you're already aiming high (appears to be aiming to the right).
Effects also get magnified or masked depending on whether your flying level, or pulling any G's. If you tilt the nose up or down, your bullets will strike high, assuming your wings are level with the horizon, for example.
Firing at the dot-target can be misleading too. Hitting in the "10" ring looks good. In realty though, that ring is 20ft in diameter. You can fit several fighter fuselage backsides in there! You actually can consider anything other than the center of the "10" ring to be a miss.
When you actually sit back and think about it, it's no wonder the "average" hit percent is so dismal. Gunnery is complicated, and it gets worse when you're moving at high speed, not level, pulling G's, etc... Understanding the "how's and why's" is really the first step in being able to get the effect you want.
