OP: Not all guns are equal, even in a given caliber. Different nations used different models of cannon, and AH reflects this. Here are a few quick and dirty rules from what to expect from your guns:
1. Any American plane with .50 cals has a very good reach, if you're very lucky you can hit targets at 600-800, but even with your convergence set to 600 yards, the bullet spread and loss of energy with distance will make an outright kill unlikely. Unless you're shooting at something that's basically a gas tank with wings- like the Zero.
2. American and British 20mm guns are modeling the Hispano cannon, which has/had a similar muzzle velocity to the .50 cal, and only slightly inferior performance at longer distances. Treat the 20mms on American and British planes like .50 cal brownings. (Whenever you fly a P-38, this means both your machine guns and cannons need the same amount of lead, so you can feel free to unload both at the same time.)
3. Everybody else's guns are a crap-shoot, both their machine guns and their cannons. A quick breakdown:
3A. German machine guns: 8mm guns in the early Bf-109s have crappy ballistics and range, comparable to British 303s. Later model Bf-109s with 13mm guns are substantially better, but these guns are not comparable to Brownings.
3B. German cannons: German 20mm cannons are not as good as Hispanos, but not too bad, either. You can get decent accurate range from them, but don't push it. Early German 30mm cannons suck- the only plane in AH with that one, IIRC, is the Bf-109G "Gustav," with the single 30mm in the prob hub. There's a reason they called it the "melon launcher," that's exactly what the projectile feels like as it languidly arcs towards the target. Late-war German 30mm cannons shoot a lot like Hispanos, they are the BFG-9000 of AH. This is why you don't get in front of an Me-262.
3C. Japanese weapons: Japanese machine guns are generally equivalent to German guns- the 7.7s are hopeless, and the 13mm guns on the late-model Zeros and the Ki-84 are better, but don't compare to Brownings (in my limited experience.) Cannons- Japanese cannons have horribad range and ballistics, it's like throwing softballs. They have a great rate of fire but very little ammo, so save it for the kill and don't try long-range snapshots like you might with Hispanos. Late-war Japanese cannons- I don't know. I think it's the same, but they might be better.
3D. Russians: Russian machine guns actually are better then Brownings. The UB has a better rate of fire AND has nice ballistics, good guns. Russian 20mms (like in the Il-2, as if anybody ever flew that,) are rumored to be great. Historically they weren't quite as good ballistically as the Hispano but were close. I don't know much about russkie birds, so ask somebody else about that. I do know that the La-7 (which seems to be the most popular Russian ride in the MA) uses 30mm cannons, and that their ballistics are rather weak just like Japanese cannons, so treat them accordingly- the point-blank Win button.
Some of this is informed more by my knowledge of historical data which doesn't exactly correspond to AH modeling, so take it with a grain of salt, and if anybody calls me a retard, they'll probably be right.