-Updated-
I. Introduction
II. Examples
III.Purpose



I. The general purpose machine gun is often confused together with the light machine gun. The two weapons classes are actually different and have specific roles reflected in their designs and characteristics.

The modern general purpose machine gun(or medium machine gun), their weight and higher recoil limits them to primarily be used as a stationary close firing infantry support weapon. The modern light machine gun is more versatile, firing a weaker cartridge and weighs less,could be used more offensively on the move handheld.

Basically, the light machine gun is the "assault rifle" of a general purpose machine gun

Comparisons-

General Purpose Machine Gun:
-fires full rifle cartridge
-significantly heavier than light machine gun
-larger
-greater recoil
-Greater stopping power

Light Machine Gun:
-fires assault rifle cartridge
-lighter
-more controllable recoil
-compact
-less stopping power

The lmg with it characteristics, makes it able to be used offensively, capable of firing from the shoulders effectively. For the gpmg, you would have to shoot with bipod mounted in stationary position.

II
A. Examples
General Purpose Machine Guns:
https://upload.wikimedia.org/wikipedia/commons/a/ad/PKM_machine_gun_in_storage.jpeg
PKM, 16 lbs(unusually light for a gpmg), 7.62x54mm
https://upload.wikimedia.org/wikipedia/commons/b/b8/M240_with_US_Army_soilder.jpg
M240(fn mag variant), 27 lbs, 7.62x51mm
http://world.guns.ru/userfiles/images/machine/mg33/mg3_01.jpg
MG3, 23 lbs, 7.62x51mm

Light Machine Gun:
http://world.guns.ru/userfiles/images/machine/mg22/negev_1.jpg
Imi Negev(FN Minimi variant), 16 lbs, 5.56x45mm
http://images42.fotki.com/v1187/photos/4/47955/6102178/RPK_right3-vi.jpg
RPK, 10 lbs, 7.62x39mm
http://www.defensereview.com/wp-content/uploads/2010/09/KAC_Stoner_LMG_5.56mm_Lightweight_Light_Machine_Gun_5.jpg
Stoner, 10lbs, 5.56x45mm
http://www.colinsun.com/Colin/nfblog/wp-content/uploads/2006/12/MG36.jpg
MG36(LMG variant of G36), 8 lbs, 5.56x45mm


By looking at the data, the distinction between the two weapon classes and role is clear

B.
MG3:
Bullet mass:10g
Muzzle velocity:825m/s
Energy:3,400 Joules

M249
Bullet mass:4g
Muzzle velocity:960m/s
Energy:1,800 Joules

The kinetic energy above is calculated using Leibniz's kinetic energy equation.

Lets also have a look a stopping power. the M249 has a muzzle velocity of approximately 3,200 ft/s (960m/s), roughly equivalent to the M16. The stopping power of modern LMGs are roughly closer to assault rifles. You can see the significant difference in stopping power between the LMG and GPMG


As the result modern LMG and Assault rifle are suprisingly similar, but heres an explanation:
https://www.quora.com/What-is-the-technical-difference-between-an-Assault-Rifle-and-a-Light-Machine-Gun

The light machine gun have roughly equivalent or slightly better stopping power compared to assault rifles. As the description shows, the technical difference is that the LMG utilizes a heavier, longer barrel with quick change barrel for sustained fire function. the longer barrel slightly enhances muzzle velocity, therefore stopping power. An RPK, is an AK47 with longer,heavier barrel for sustained firing.

III. Purpose and My Proposal:
1. To separate the two weapons classes, fix the erroneously classed weapons.
2. Replace the Imi Negev with the MG3 as the standard Grey machine gun. The MG3 is the successor to one of the most successful machine guns of WW2, the MG42 and MG34, the guns that set the definition of the modern general purpose machine gun. The MG3 is the identical version of the MG42, the only difference being that it is rechambered in the 7.62x51mm NATO cartridge. It is so surprising that the gun with high reputation is not included in the game.
3. Add the RPK & MG36, set the lmgs into another category. deciding rare status is up to the decision of the Devs



Sources:
http://www.globalsecurity.org/military/systems/ground/m16-specs.htm
http://www.globalsecurity.org/military/library/policy/army/fm/3-21-9/appb.htm
https://books.google.com/books?id=VWkYoAkoMHIC&pg=PA225&lpg=PA225&dq=mg3+muzzle+velocity&source=bl&ots=HFx_qJgwn5&sig=K_7n9xQ-Aq1xr2cNCWvQkpRnsfI&hl=en&sa=X&ved=0ahUKEwiIzq_Bj63KAhWDeD4KHV9yAmk4ChDoAQgbMAA#v=onepage&q=mg3%20muzzle%20velocity&f=false

Love this idea . The Stoner is already a rare in game but I do think there needs to be some variety in the weapons.

A little off topic but, I love your layouts and neatness with your proposals and ideas

Thats how I make complex proposals

When you want to go into detail, you want to provide introduction,stats, and main proposal in a presentation combined with organization and neatness.