It was the western version of the AK-47 being that it fired a fuck-off massive bullet and was rugged as hell. If you want it as an AK-47 replacement just have the Soviets copy it and start churning out knock-offs that they give to their allies. I mean it's not as though they were ever great respecters of international patents or copyright if it didn't suit them.
Because a rifle/gun is always built with a "theory of war" and context of use inside its design. You're looking at the two guns, AK and FAL in a vacuum. Consider the time, the soldier it was meant for, the way and conditions or the army it was designed for, how they had to fight in and the resulting type of soldier it went to. You can even think about how easy it was for developing nations to produce and maintain them. Any guns can and must be analyzed according to its "theory of war" and context of use.
- The FAL is a perfect example of a Western military thought battle rifle. It was accurate, powerful enough to nail any enemy soldier you can see, more expensive to manufacture, designed for professional soldiers well trained in individual semi-auto marksmanship.
- AK-47 was cheaper and simpler, easy to mass produce and reliable. Soviet troops and later guerilleros weren't trained in semi-auto marksmanship. It emphasized mass and the whole rather than the individual. It was an assault rifle.
They had their own advantages and their own defaults from different categories :
- FAL = longer range, bigger bullet
- AKM = lighter, smaller, and larger mag capacity
We could also speak and debate about engagement distance and ammo size. If you fight at ranges under 25 yards, the assault rifle has an edge. Between 25 and maybe 100 they're about equal. At extended ranges or through hard cover, the battle rifle has an edge. Another problem is the weight of the ammo. If you have to carry 500 rounds of assault rifle ammo and 500 of battle rifle ammo, guess which weighs more. Was the big bullet of the FAL really useful?
By the way, the Israelis were armed with FALs in the 1960s. They found that they were excellent for long-range shooting, but that they were prone to failure in sandy conditions. An FN-FAL might be a great rifle for western Europe, but they're not so good in the desert and probably not in the jungle. FAL was the equivalent of the American M-14. The FAL has several good things going for it, including the adjustable gas system and the low centerline of the bore (in relation to the receiver). It is a robust design and very reliable. It is, however, expensive to manufacture and relatively complex in assembly.
The AK fit the role it was designed for perfectly. An utterly reliable (even at the cost of accuracy), easy to use rifle that could lay down a lot of lead fast. The FAL is perhaps a better rifle all around, but considering the AK's intended purpose I wouldn't use a FAL for it.
Your scenario is possible, but the guerilleros would need to be better trained and wouldn't be able to use them the same way AK-47 was used, which would create a lot of butterflies.
-> Soviets would use their best designs as close as possible to AKM :
- Korobov-tkb-40 http://world.guns.ru/assault/rus/korobov-tkb-40-e.html
- AB-46 by Bulkin http://survincity.com/2010/08/the-main-rival-of-ak-47-on-the-competitive-tests/
- AD by Dementiev (no link sorry )
