I never understood the difference between machine guns, mini guns, gattling guns or lmgs. Doesn't lmg just mean light machine gun?

LMGs fire rifle caliber rounds at a high rate of fire. HMGs fire larger rounds, (think .50) a little bit slower than assault rifles. Gatling/miniguns use a group of rotating barrels to achieve a extremely high rate of fire.

Which ultimately means that the designation of "Machine Gun" in Destiny is a misnomer. Generally, the difference between an LMG and HMG (when caliber is removed from the equation) depends on whether or not the weapon is portable. The "Machine Guns" in Destiny obviously are, hence my use of the ever-so-slightly-more-accurate term of "LMG" in some instances.