So realistically I doubt they would ever resort to fully auto fire in the first place.
Depends on what they're shooting at. A terminator would be a target worthy of full-auto (which a squad of Marines Errant manage kill a terminator in Blood Reaver
). Or a Squiggoth or some large tyranid creature.
Firing at extremely fast moving targets (like the Void Stalker in, er, Void Stalker
Would a Space Marine be typically firing on full-auto? Very unlikely. But remember Space Marines, like their war gear, were designed to take on anything the galaxy could spit at it. They were also designed for massed warfare--as we see in the pacification of Terra, the Sol system, and ultimately the Great Crusade.