This is a timeline on how various weapons turned out to be the way they are!
April 17, 1961: A 4 man strike team of M16s assaulted a villa held by the FN FAL forces. They luckily were able to recruit some FN FAL rebels to assist them in the fight, along with some Skorpion mercenaries. At the time, the M16 team had a deal with the ASPs who supplied during the mission. While successfully assassinating the FN FAL leader, they weren't expecting RPK soldiers to be facing them. The M16 team barely escaped with their lives onboard an M60 plane.
November 17, 1963: A strike team from the MP5K team attacked an AK-47 held cosmodrome. Backed up by weapons from the Python coporation, they managed to kill many hostiles, including some recruits from the Dragunov and PM63 factions.
January 21, 1968: Deterred by recent defeats, the AK-47 team decided to attack a large M16 and M14 stronghold. M16 and M14 had been in partner ship for a long time and as such had recruited many other people to their cause. During the attack the AK-47 army was backed by the RPK soldiers. Despite a heavy attack, the AK-47s and RPKs were driven back by a joint force of M16s, M14s, Stakeouts, M60s M1911s, Pythons, M72 LAWs and China Lakes.
February 2, 1968: Meanwhile other RPK soldiers has been busy. They had joined their old allies, the FN FALs in attacking a city held by Commandos and SPAS-12s. After much heavy fighting, the Commando-Spas-12 Alliance won, but not without assistance from some M60 soldiers. Right now, the M16s and Commandos were dominating the field.
February 9, 1968: A real shitstorm happened here. At peace talks, CZ75 and Spas-12 soldiers found them selves attacked by a huge force of PM63, Spectre, Kiparis, HK21 and Dragunov forces. However the situation improved for the CZ75 and Spas-12 soldiers when they were assited by G11, PSG1, and China Lake soldiers.
February 9, 1968: During this time, Commando operatives snuck up and attacked an AK-47 and RPK held village. The intial attack was a success, spelling another defeat for the AK-47s. However the Commando soldiers also had help from the WA2000 men, M1911 troops and Python soldiers. During the battle, the AK-47s gained support from Skorpion mercenaries.
February 11, 1968: On the run from the Commando team, the AK-47s and RPKs teamed up with a new faction the AK-74us. But not even they and the Dragunov team save the AK-47s from defeat at the hand of the Commando army, backed up the China Lake side and the M1911 faction.
February 18, 1968: While this had all been going, two other factions rose to power: The AUGs and the FAMASs. Now the FAMAS had managed to take over the AK-47 side, promising them victory. They were influential people, as they were able to recruit the Galil faction, along with HK21 soldiers. Even the Spas-12 and CZ75 side managed to strike a deal with them. Skorpion mercenaries were also present on the FAMAS side. But all those were no match for the AUG special forces. Along with M1911 men, they managed to destroy the whole facility.
February 19th, 1968: Ever since the FAMAS defeat, tension had been running high between the Galil and the AK-47 side. Eventually the Galil side arrested the AK-47 soldiers. Even their once faithful allies, the RPKs joined the Galil. This would be a bad move as the AK-47 soldiers and CZ75 men would escape and over-run the facility. Even the new guys on the block, the Uzis and Olympias couldn't stop them. This would be a triumphant victory for the AK-47s who had suffered such heavy losses previously.
February 23, 1968: Meanwhile, two factions had become big enough to pick a fight. The new Enfield faction along with the HK21s attacked a base held by AK-74us, RPKs, and Kiparis soldiers. After this, the AK-74us retreated to their base on a ship.
February 26, 1968: Endgame. The FAMAS men, backed up by a MAC11 division, attacked the last stronghold of the AK-74us. After heavy fighting, they were able to finish off the very last Galil, RPK, CZ75 and AK-74u forces.
Epilogue: After this, the factions would spend years fighting amongst themselves, only to be replaced by newer regimes.
Additional notes: RPG soldiers fought for everyone during the whole war.