There are two issues at hand behind what's making the 50cals seem "weak" to some people in AH;
* The first one, is misconception.
50cal machine guns primarily depend on kinetic impact for damage. While a few of the rounds may contain some HE content or incendiary qualities, it is hardly comparable with a contemporary cannon round in terms of destructive power. Ofcourse, the .50 round is massively potent, and will puncture through the plane's skin easily above extreme ranges.
However as long as it depends on kinetic impact, it needs to hit something vital inside the plane to bring it down. This requires a concentrated burst of fire to land on a single spot, maximizing the firepower and the chance to hit vital internal machinery. If the shots are spread out, they may not do much more than drill a 50cal sized hole on the plane's skin. Cannons are very different. They are essentially small grenades lobbed at high speeds. The are designed to detonate on impact, ripping out entire surfaces of the plane's fuselage, wings, stabs, etc.. instead of relying on hitting anything vital.
In many cases in AH, people either misjudge the amount of damage they've actually done, or have problems in landing concetrated shots.
* The second one, is an inherent problem in the DM.
This is all speculation, since HT never confirmed how the DM works.. but if we continue, AH's damage modelling does not model internal machinery, and the damage is registered as "all or nothing".
Imagine a real life situation where an enemy plane is hit with a deadly barrage of .50 rounds all over it. Like a miracle, none of those rounds have hit anything immediately fatal - such as the fuel tanks, engine, or the pilot. However, the chances are that the 50cals did hit something that might make it very difficult for the pilot to keep his plane in the air.
For one thing, it would have punched numerous holes on the plane, effecting its drag and slowing the plane down a bit. If the control surfaces were made of canvas or so, it would be severely tattered and would be rendered extremely ineffective. The rounds could have hit important internal cables or rods that move the ailerons or the elevators, robbing the plane of mobility. All these hits may not be immediately fatal, but the are certainly potentially dangerous.
Now, AH does have a few internal machinery depicted. It has engine damage, busted oil cans, and leaking fuel tanks, destroyed guns and gears. However, as long as the pilot and the engine is safe, there's nothing that can be damaged "internally", that will bring the plane down. You can shoot all you want at the target's wings with the 50cals, but if the total amount of damage done to the wing is not enough to snap it, then the plane's performance is undeterred.
This means that planes in AH are shot down by structural failures predominantly. You can rake across the enemy plane from wingtip to wingtip with 50cals, but as long as the pilot and the engine is safe, the target plane is utterly unharmed.
.............
Therefore, I believe the consensus is that AH 50cal rounds are functionally accurate... but ppular misconceptions, as well as actual DM flaws, have made the 50cal seem 'weak'. I don't think the 50cal is weak.. but I do think the DM needs to be changed.
