i disagree completely with the first post, no offence but fallout has its own style, not our worlds.
im thinking the ability to complete a qeust for a techy guy to get a pheromone weapon, where you apply it to yourself to temporarily hide yourself from that faction, also every weapon ( made or not ) from the fallout universe
We had the Deagle in FO1 & 2, the P90 in FO2, the FN FAL in FO2, the M16A1 in FO2 and a gun looking very much like the L-85 in FO2.
Most of those where made after the 50s. So more modern weapons is not out of Fallouts style.
And we did have the Chinese Assault Rifle in FO3, which was practically a AK-47.