alt Hacker NewsThe USA mostly used .50 caliber machine guns, usually with a mix of ammunition including incendiary bullets so that a hole in a fuel tank meant a large fire. Fighters from the other major combatants usually had 20mm autocannons in addition to smaller machine guns.
Allied fighters were also equipped with self-sealing fuel tanks, so a hit doesn't automatically mean it burns. I don't have any stats on it, but they wouldn't have added the self-sealing if it didn't improve the survivability.
The sensitive part for a P-51 was the cooling system. Any hit on that, and you're done.
B-17s famously endured a lot of battle damage. The usual vector of attack on them was head on, and they aimed for the cockpit. (Attacks on fighters usually aimed for the cockpit, too.)
I know that tracers were used in WW1 to set observation balloons (filled with hydrogen) afire. Tracers in WW2 were used so the gunner could direct his aim. I haven't read that they were intended for the fuel tanks, but that could be true.
109's would frequently sneak up from the rear, and if the tail gunner was not paying attention, it was an easy kill. My dad (B17 navigator) said the tail gunners, once they spotted a 109, would fire a few rounds of tracers long before the 109 was in range - just to let the pilot know they were awake and aware. It usually meant the 109 would veer off.