The name NeuSturmgewehr would suggest that the NSG 23 is of German origin, perhaps designed by a German subsidiary of Weyland-Yutani. The term Sturmgewehr ("storm rifle", or, in other words, assault rifle) was developed by Germany during WWII and was applied to what is widely regarded as the very first assault rifle, the StG 44.
Given the above, the use of "Assault Rifle" in the weapon's name is superfluous, as the translation of its full name would be "New Assault Rifle 23 Assault Rifle".
The "23" in "NSG 23" could be a reference to the date the weapon was created (which would likely be 2123, a year after the events of Alien).
Despite "NSG" and "23" being separated everywhere else, in the DLC level Firing Range, the NSG 23 inside the locker is known as the "NSG23 Assault Rifle".
In the Brady Games Aliens: Colonial Marines Guide, "Assault Rifle" is misspelled "Assult Rifle" in the W-Y Grunt section on page 40.