The writing sounded familiar, and he used a 20mm to take down a chopper in his book.
John has had a 20mm Solothurn S-18/1000 since his high school days, and not surprisingly chose it as the weapon of choice for his Unintended Consequences protagonist.
The German mount for the Solothurn is additionally equipped with wheels, perhaps handier for the quick getaways of John's novel than the ski-mount skid feet of the Lahti. Unless it's snowing real hard.