The Soviet light machinegun concept was a simply one, simplified even more when the moved to the RPK. The original ‘new’ soviet post-WWII infantry layout was going to be the SKS as the primary, the AK replacing the PPsh sub gun types, and the RPD as the squad light machine gun.
All ate the new 7.62×39 M43 ammunition.
Logistical life was grand.
Then further experience indicated that the AK was the superior choice to the SKS and eventually the RPD got the magazine fed treatment too with the RPK. But the RPD still stands as a classic example of good design with an emphasis on logistical ease. The PKM was built with that same mindset and is still one of the three worldwise formats for GPMG’s worldwide.
Now, if you are wondering what differentiates a light, medium, general purpose, and heavy machine guns. Let me break those down in a simple unofficial way.
Light Machine Gun
Single troop portable.
Single troop or two troop team fireable, preferably single.
Effective range that of the infantry rifles it is supporting, usually the same caliber.
Medium Machine Gun
Single troop portable for the gun, supporting equipment (tripod) requires a team.
Two troop to three troop fireable machine gun team.
Useful as an emplaced gun.
Caliber and effective range usually greater than the infantry rifles it is supporting.
General Purpose Machine Gun
Configurable for emplacement, infantry movement, and vehicle mounts both air and ground. One core gun for many duties.
Usually a medium or heavy machine gun.
Heavy Machine Gun
Chambered in a caliber capable of anti-material, vehicle, and usually light armor defeat.
Infantry team is usually 3 minimum for both transport and employment.
Vehicle and emplacement deployment is most common.
Effective range comparable to vehicle line of sight weapons, including grenade launchers and some light cannons.
Keep in mind, these are generalities. A gun design can easily start bridging into multiple. The new NGSW submissions have a light machine gun that outperforms current generation medium machine guns and nicely set up as general purpose too. The new medium machine gun in .338 has the effective range of a heavy while being as portable as a medium.
We may see future tech merge the heavy machine gun with auto-cannons and retiring the concept of a heavy, clunky, team mandated platform line-of-sight kinetic only automatic weapon entirely. The new MG338 is offering a lot to the infantry and GPMG needing platforms while also being very easily pulled from one role into another at immediate need.
Need to pull the MG off a truck and into a building? Just grab it and go, the stock is still on and just folded or in a bag. Need to move it from a truck to a helicopter pull it, carry it, remount it. With the ammunition modernization efforts, combined with the refinements in weapon design of the last 30 years, we are reaching precision capabilities that were undreamed of when the RPD hit the fields.