That's Fallout 2. Weapons in Fallout 1 were designed by Chris Taylor. Adding RL firearms instead of new fictional ones or leaving the weapons alone was the result of someone else doing it. Chris Taylor only wrote the manual for Fallout 2.
Anyway, modern real life weapons like Jackhammer, G11, P-90, CAWS etc. popping up just before the war doesn't make sense. They were a product of their own world and times. Especially that Fallout's world has it's own weapons - it already has a Winchester combat shotgun, a micro-calibre assault rifle, etc.
Both FN FAL and M3A1 are justified because they are pre-divergence weapons. Still, the Assault Rifle is already an old weapon that was out of use around the time of war. Finding large amounts of FN-FALs doesn't make much sense.
Still, one thing that could be neat would be re-emergence of old designs - for example people like Gunrunners could make WWII/50s era guns just because they don't have technology to make more advanced ones. Sadly, the opportunity was missed and they were making super advanced weapons in a workshop that didn't even have a power source (fucking Feargus Urquhart
)
The time just before the war was a time of energy weapons. Assault Rifles were already out of use. Anyway, if they wanted to add new firearms there is some space, but not for the modern RL guns.
For example, Assault Rifle still is made of wood and metal. It seems to be a very old design, probably from the XX century or the beginning of the XXI century. There could be another assault rifle designed before the Energy Weapons era that could be a companion to the Combat Armor and Combat Shotgun. It could be bullpup and named Combat Rifle, or something like that. But they didn't bother to do design and speculation, they just inserted RL guns.
One thing that I find amusing is that the expectation for fictional guns from the sequel was so strong that I thought that all the stuff like FN-FALs, G11s, etc. is fictional just like the Vindicator Minigun and Gauss Rifle. I was very surprised to learn that these are real guns.