>>64056412There is little real difference between these outside of ad hoc nomenclature. One gun could be called something in on military and another in a different one.
Automatic rifle, theoretically, refers to magazine fed weapons that are designed to fire full auto all the time. But then some belt feed machine guns are called automatic rifles.
GPMG usually refers to one MG design that is meant to be adaptable to a variety of fixtures easily, its very modular. So you can carry it, or mount it in different ways and the overall body of the gun is adaptable for that. But then some are more modular than others in a substantial way.
LMG vs SAW arbitrary. SAW just came from a specific name that the US used for a program ran in the 1970s that has no larger meaning outside of that to differentiate it from LMG. SAWs are LMGs.
LMG vs MMG is again, very mutable. The first LMGs were just lighter guns made in full power rifle cartridges, today they're often in intermediate calibers but some aren't. MMG used to mean just a heavier rifle caliber machine gun, more likely to be fixed in place and rarely carried, but then came to mean full power rifle cartridge firing MGs of all sorts, and some are very portable today.
MMG vs HMG is again, mutable. HMG just means a gun is too heavy to run around with, usually. Caliber is irrelevant. But then some HMGs, like the Chinese .50 cals and some experimental American ones, can be carried around easily by one person.