The Barrell Roll is a trickshot that can be done in Call of Duty: Modern Warfare 2. It involves the players' shotgun doing a sidewards or backwards roll. This can only be done with the SPAS-12 and the M1014. It is possible to do this trickshot in mid-air which is what more advanced trickshotters tend to do.
To do the Barrell Roll, you double tap triangle or Y really fast whilst sprinting forward a little bit just before the shotgun comes out when switching to it.