Given that by 2000 WW3 is several years old I suspect that a large number of civilian and export weapons will make their way into official use. The USA has a huge domestic firearm manufacturing industry and given what happened in WW2 many will start making weapons for the US military and its allies.
Also individuals may bring or purchase private weapons before being deployed. Hence I'd expect to see a lot of civilian pistols and revolvers, SMG and AR-15 type weapons being used by military personnel. Add to that weapons from allies and captured enemy weapons and you'd have a wide range of weapons available to the Characters.
Of course we don't have the stat blocks in the rules yet but it is possible to create them using existing weapons.