An experimental pre-War weapon produced from high strength polymers and almost indestructible as a result. It's lightweight, accurate, and has a high rate of fire, while maintenance is made easier by the fact that it can be stripped without any tools.
A superior replacement for the AK47 or the M16A1 when it comes to damage. It does have a smaller capacity than either and much shorter range than the M16A1, which can fire bursts at a farther range than the XL70E3.
St. Louis: In the mutant base, in the southwest corner, inside a crate near the moat.
Behind the scenes
XL70 was the forerunner of the SA80 assault rifle family (used in service as the L85). It was distinguished by its use of 5.56mm NATO ammunition, the first of its kind, as the preceding XL64 and XL65 used an experimental British 4.85x49mm bullet.