I am not a member out at Flightsim. Actually I probably AM a member but can't remember the login or password.
Here is the "Weight Issue" as I see it. Most fighters eject the spent case and link. Some do not. I believe the 37 mm cannon on a P-39 does not. Typical swivel guns do not toss the spent cases overboard. They end up on the floor of the aircraft. In this case (no pun intended), the weight of the ammunition should be taken into account when the aircraft is loaded. It is carrying the extra weight at Take-Off. The weight is not reduced when the ammunition is spent. I don't know we can do about that in CFS. I say that we should put in the full weight of the rounds and links so that ammunition loads are a distinct weight factor in performance. I go to a fair amount of effort to calculate the Basic Weight plus pilot.
BTW, Why are German pilots heavier than US Pilots by 20 pounds??? US Pilot weights are 200 pounds. German pilots are 220 pounds (100 Kilograms).
With US fighters, often there is a selection of load conditions: Fighter, Fighter Bomber, and Fighter Overload. Funny thing, but as a "Fighter", the aircraft typically won't be carrying a full ammunition load or full internal fuel load. Fighter Overload has both at maximum.
Also, I am not sure where you got the numbers for a .50 caliber round. The full round and link weight which I got from the P-50C loadout was 760 rounds == 228 pounds. That works out to 4.8 ounces per round.
Regarding the damage from a .303, I really don't know. I believe they are too potent in the game. Some of the cannons like the 20 mm MG 151/20 and 30 mm MK 108 are not potent enough. The statistics I quoted were gathered by the Luftwaffe. If the AVERAGE number of hits needed to kill a B-17 is only 3 rounds, there were plenty of cases it took fewer. Check out wartime photographs for confirmation.
I make no claims about a .50 caliber because I haven't done the research, but I would make an assumption that a single MK 108 hit should blow up the Jeep's fuel tank, right?
When flying against AI bombers, you would think you are flying against B-40s. Their gunnery is intense and extremely accurate. A single pass doesn't tend to kill all that often in my experience though perhaps I don't shoot very well. The following is yet another diversion from the topic, but I believe the bomber DP files should have reduced range for AI gunnery. You can't much alter their accuracy, but a fellow with a ring and bead sight or just a ring sight (often USAAF swivel guns didn't have a front sight), isn't terribly accurate at any range and certainly not past about 200 yards and certainly not to a thousand yards.