There were glimmers of "light-weight, personal firepower" one hundred years before the M16 was developed; and while a perfectly workable 1st generation aluminum-framed, assault rifle could have been produced with TL4 industry, no one foresaw the need of such a weapon as no one foresaw the realities of industrialized warfare.
It is worth noting that, in the case of current assault rifles, while they may be lighter and not be capable of killing at two kilometers, they are not using "carbine" ammunition anymore; at least not in current non-Traveller terms. The shortened rounds of the Stg-44 and AK-47 are old tech, while the 5.56x45mm and the equivalent Russian round are full-power "Light" rifle cartridges, man-accurate out to the same ranges the rifles of the early 20th century were.
It is worth noting that, in the case of current assault rifles, while they may be lighter and not be capable of killing at two kilometers, they are not using "carbine" ammunition anymore; at least not in current non-Traveller terms. The shortened rounds of the Stg-44 and AK-47 are old tech, while the 5.56x45mm and the equivalent Russian round are full-power "Light" rifle cartridges, man-accurate out to the same ranges the rifles of the early 20th century were.