4x30mm will trump 6x .50 cal just about every day of the week except Sunday, and only then between 11:59PM and Monday.
In-game. It has been discussed to end before in these forums that in-game ballistic characteristics of our MK108 30mms are almost lazer-gun acurate compared to real-life. I forget the hard numbers, but it was something like 15-25 foot random hit-box at 200-300 meters would more acuratley represent the German's 30mm balistics. They were used in the real world more in a fashion of spray and pray, here in the game we use them as accurate slow-velocity high-drop-rate potatoes. But similarly to the ones modeled in the game, when and if they made contact with an enemy AC in real-life, boy did they make a noticable contact.
In a dogfight between a P-80 and Me-262 I'd imagine the gun solutions would be mostly brief snap-shots, 1-2 seconds of pulling the trigger, and in that situation I think a P-80 would be stronger as it may not kill the 262 in one sucessful solution, but it could damage it and then put it in a situation to get sadled up for a solid kill. With the 262, it would be hit or miss, either he's gets lucky and lands a shot that most likely puts the P-80 out of the fight, or he misses repeatedly, and given the information provided in this thread it seems the longer a Me-262 has to hang around and fight a P-80, the less likely it is to fly away the winner.
So, if this discussion isn't to take into account any AH factors, then it isn't a far-fetched conclusion to draw that 6 balisticly acurate .50-cals with a lead-computing gunsight would give 4 balisticly accurate 30mms with a revi sight a very good run for theirs.