First, with .50cal your convergence setting is VI-TAL. .50s are are just slugs, not explosives as in cannon (or ARE they modeling API in the Ma Deuce?). To do any significant damage you HAVE to put more of them into the target. This is where convergence comes in.
Aircraft like the P-51 and F4U-4 have the guns out in the wings, so they have to fire inward at an angle. Your point of convergence is the distance at which the bullet streams from the guns intersect, and that's your optimal firing range for maximum effect.
.50 cal lack the energy of 20mm over range, so you want them centered in close. In the F4U my guns are set to converge at 300yds distance for air-to-air combat (I have them set longer for air-to-ground). Too much further out and you lose a good bit of hitting power. Next comes the type of convergence. Some people will argue for a zone (each pair of guns centered at a different range, ie 275/300/325yds for inner, middle and outer pair respectively) but I prefer a point (all guns centered at the same range) which has all six guns hitting at the exact same location. This increases your hitting power, and even short bursts in convergence range WILL knock out most fighters if you hit them in the right spot.
This means you MUST watch your range. You can make the long shot, but don't rely on it to do much other than maybe get your target to evade and allow you to close in for a kill shot. Wait for your target to enter your convergence range FIRST (I seldom fire unless the target is between 200 and 400yds distance).
Without more information, my guess is that's your problem: Bad convergence settings and possibly firing at too long a distance. If you can hit with cannon there's no reason why you can't score hits with the Ma Deuce. The .50cal is probably the most point-and-click air-mounted weapon in the game, and within its killing zone has the better ballistics and the higher muzzle velocity means you need less lead than you do with cannon. I generally put the pipper a little high to account for drop when I fire.
As to aircraft:
The Corsair is NOT an EZ-mode plane like the Spits and Las. It takes a bit of finesse to get everything you can out of her (see this thread:
http://forums.hitechcreations.com/forums/showthread.php?s=&threadid=198481 ) but if you learn how to take advantage of her assets she's one of the best all-around fighters in the game (the -4 is arguably THE best. PERIOD).
The key advantages of the F4Us are:
Speed -- Especially the -1, -1A and -4. The -4 is among the fastest prop aircraft at all altitudes (she WILL outrun the P-51 at most altitudes). At full steam she'll run down or escape most anything that out turns her.
Rate of roll -- Especially at speed and light of gas the Corsairs have an EXCELLENT rate of roll. Use this to advantage to reverse on opponents and keep them out of phase and unable to get a shot. The rate of roll improves with speed, so you if you can sucker most opponents into a high-speed dive you can roll yourself out of position while they're fighting unresponsive ailerons.
Firepower -- I don't care what anyone says about them in comparison to cannon, the .50cal are LETHAL. As I said above it's a matter of firing range and convergence.
Durability -- The F4U can take a beating, although I still think some parts are under-strength and made of glass (*cough*engine!*cough*). .30cal won't do much unless you let your opponent get a LOT of them into you at the right spot within convergence, (it reminds me of a gentle summer's rain drumming on a window pane) though even a Spit I or Hurri I can maul you pretty good if you let him get a good tracking shot on you. .50cal of all types are more of a concern. It DOES generally take some pretty concentrated fire to bring down a Corsair in one burst so if you can keep your opponent from getting a good tracking shot you can generally take a few hits and keep going. Cannon are a different beast entirely, but very few aircraft can take more than a few shells, anyway.
Energy Retention -- The Corsair is HEAVY (weights near or exceeding 12,000lbs under some fuel and ordinance loads) so she really holds on to energy well. VERY few aircraft will catch the F4U in the zoom climb (most likely exceptions are the Tempest and 109K-4. Even the La-7 will find it hard escaping--or catching--an F4U in the Zoom). NEVER overlook a Corsair below you, because if you've misjudged his energy level that Hog can be above you in a second.
Energy Dumping -- That said, the Corsair can dump energy VERY rapidly if she needs to get slow. The landing gear can be kicked out anywhere under 400mph, and she has a big and very effective rudder. You can surprise a LOT of Spits by dumping E and flaps to cut inside them.
Rudder -- As stated, the Corsair has an EXCELLENT rudder. Get yourself a set of pedals and work on coordinated turns. That big rudder can help you shave off a few extra feet in turns, shed E in a hurry, and in vertical fights swing the nose around to reverse on a climb, and maybe even drop into a shooting solution on the target trying to pull under you. You can drop one hell of a nasty surprise on guys if they think they're passing out of phase to one side of your guns by kicking the rudder hard and skidding your nose on 'em.
Der Uberflappen (TM) -- I don't think ANY aircraft in the game has better flaps than the Corsair. One or two notches of flaps timed correctly, especially with careful throttle control, can really haul that nose around, and I've surprised more than a few Spit drivers with a last minute lead shot just as they thought they were escaping behind me. The Corsair's turn radius is almost on par with the Spitfires with full flaps engaged. They'll really help you through turns and loops, so you want to make knowing when to drop your flaps as near to second-nature as you can make it.
Instantaneous Turn -- The Corsair is superb in its initial turn, and as above with careful management of flaps and throttle, you can often catch aircraft with superior sustained turn before they can take advantage. I think the F4U corners right around 200-250mph, but she's also HIGHLY maneuverable at high speeds, something that CAN'T be said for most aircraft that will out-turn her in the lufberry.
Dive -- The F4U can drop out of the sky like a lead block with wings. She'll be outpaced initially by many opponents (see below) but once they lose their initial acceleration advantage she'll outpace almost anything in the dive: the 1A especially can really haul bellybutton with the nose down. Plus, you can expect to maintain control at some EXTREME airspeeds which is more than you can say for almost anything else in the plane set. In a little offline testing I was able to maintain control well above 550mph despite a wicked shimmy. Although the elevators were barely responding by 600, she still hadn't entered total lock when I smacked into the ground (this was in a near-full vertical dive from 24,000ft, and there was a mountain in my way. I don't know if I'd have pulled out if I was over water, but it's impressive I still had some elevator control and WAS pulling out even at that speed. If I had started my dive at less angle for a longer run I may have gotten level, and possibly gained even more airspeed. I hit 600mph and the ground at roughly the same time). I didn't shed so much as a control surface, either, and these are BEYOND speeds where many aircraft will either compress entirely or begin falling apart. So keep in mind if you've got some room to spare and need to get out of dodge point that big nose at the ground and goose the throttle.
