Nitro, actually the over under doubles are far more mystifying than side by sides.
Believe it or not the o/u bullet paths spiral around each other and therefore cross over many times.

This of course will only work on 2 trigger guns when you snap both locks at precisely the same instant, and their path could best be described as similar to a corkscrew!
Try this yourself with a eight pound 500 Nitro O/U and you will quickly sense a blurred, dizzy, rotating, head spinning sickly feeling, sometimes accompanied by visions of Elvis or John Lennon.

Southern hemisphere o/u guns usually have their spirals rotating in the opposite direction to o/u's used above the Equator.