Ah I love this question. Originally, all branches of the UNSC used the M392 DMR as its utility marksman rifle. However, when the BR55 came to the market, it surpassed the M392 DMR. As a result, the DMR was phased out of use in all branches but the Army. (Noble Team falls under the jurisdiction of SPECWARCOM which is under UNICOM, the Army.) Fast forward to Halo 4, and a new model of the DMR has been introduced. The M395 DMR. Presumably, it improved on any shortcomings its predecessor had, and was reintroduced back into the UNSCDF as a whole.

Thanks, man