It's all a matter of numbers, really. The first round out the barrel is just as effective as the last. The problem really is that you tend to run out of your main ammo first (cannons) and those rounds are more lethal than the secondary guns on most planes. Many people barely even fire the secondary guns and only rely on the cannons to get kills.
If you are low on ammo, you are often most low on cannon ammo and are relying on lesser calibre weapons to get those final couple of kills. In the case of a .50cal, you have a fair chance, but with .303's like on the Spit V, well, you have your work just out for you.
Then again, anytime you have less than 50 or so rounds you should be returning to base. Even with a hit % around 10, only 1 in 10 of those rounds are going to hit the aircraft. More often in the case of a newer player you'll be talking about a 2% hit ratio so you are talking maybe 1 hit in 50 rounds. 1 hit just isn't going to get the job done. Many times an aircraft can take several hits, especially from lesser calibre weapons, before you do any damage. Chances are you should be looking for a way out when you hit 100 rounds and use those last shots to aid your escape.
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Soda
The Wrecking Crew