In the books it’s kinda a mix of dumb luck and Sharpe being the equivalent of a British Terminator, who does nothing but kill and sling dick across Europe and India.

If I remember correctly, among other normally mortal wounds he gets shot in the dome twice throughout the series and lives.

The story kinda jumps the shark a bit here and there, but the descriptions about the military history like logistics and tactics are pretty great in all the books.