The Assault Rifle is a scrapped Duke Nukem Forever weapon.
The Assault Rifle automatically fires bullets like a normal assault rifle, but in the 1998 trailer, it also had a pump action grenade launcher and a usable scope. In the 2001 trailer, the scope has a small American flag tied onto it.
The Assault Rifle was also supposed to be used by EDF soldiers, as seen in the 1999 Unreal Engine 1 screenshots and the 2001 trailer. The EDF variant had a grenade launcher, but no scope.
All design phases of the Assault Rifle can be seen in various trailers and pictures, even the sounds, scope...
1997-1998 Assault Rifle had different sound model and animations than the other versions.
1999 Assault Rifle was similar in the design like the 1998 one.
2001-2003 Assault Rifle was siminilar to the 1999 version but has some differences. EDF from 2001 were the same like from the 1999 version and still used the 1999 Assault Rifle.
2004 Assault Rifle was futuristic looking, and looked like the Unreal Tournament 2003 Assault Rifle, it had a completely different design, but still had the american flag on it.
It's known that 3D realms scrapped the Assault Rifle in 2004.
The Assault Rifle was eventually dropped from the final game build, being replaced with the Ripper. This was due to the personal preferences of the development team, who wanted to see the original Duke Nukem 3D arsenal rendered in full 3D.