In an effort to bring ancient technology more up to date with modern weaponry, the DC-15 'L' derivitive was created by the Grand Army of the Republic to allow their soldiers better advantage on the Legacy-era battlefield. Designed as a maverick enterprise by members of the Engineering Corp, the L class DC-15 rifle bears many of the hallmarks of the original DC-15A, but is slightly shorter, much lighter, and much more powerful due to the advanced miniturization, new-age materials, and more powerful payloads respectively offered by the passing of the centuries.
The DC-15L weapon was never officially approved for use by soldiers in the Grand Army, being designed and manufactured under the guise of a research project by engineers, but after its creation the weapon was quickly adopted by the more elite members of the Grand Army into their standard arsenal. The Clone 501st itself receives the weapon standard-issue beside their own DC-15A models, and due to the far more advanced design offered by the 'L', the latter tends to gather dust in the 501st armory.
It is unknown who approved the expenses of the DC-15L's creation or why, but it is rumored among GAR soldiers that the project was funded entirely by a mandalorian investor, a rumor fuelled by the fact that the weapon manuals for the DC-15L are issued only in Mando'a and not Galactic Basic like its predecessor.