I think they used that new one cause they wanted it to be big looking even when held in the hands of a power armored character.the New Vegas assault carbine or Fallout 3 G3 Assault Rifle would have looked a bit under-sized.thankfully there are some great mods that add back some older guns.for example theres now a New Vegas assault carbine mod, a Fallout 3 Assault Rifle (G3), a New Vegas Service Carbine Mod (with the unique Survivalist's Rifle from Honest Hearts), 3 seperate Fallout 1/2 Wattz 2000 laser rifle mods, a New Vegas Hunting shotgun mod (with the unique Dinner Bell), a New Vegas Battle Rifle (M1 Garand, with the unique This Machine) mod, a Fallout 3/New Vegas 10mm SMG mod, a New Vegas 9mm pistol mod (with unique Maria-esque pistol), a New Vegas.
