U.S. Army - Choosing/Using Weapon other than M16?

In some cases in some units the Marines will allow you to use a military caliber personal firearm as a secondary weapon. Some Army units were doing it but I think that was stopped but not sure on that; my niece in the National Guard was allowed to take her 9mm Glock to iraq but still had a M9 issued. The Marines were approving on a case by case basis some personal weapons (typically m14 variants) but that was usually higher ranking enlisted (Staff Sergeatand above) and Officers. The lower ranks were not typically approved as the civilian versions of the rifles in 5.56 do not have the capability of automatic fire (full or burst) and have no advantage over the M-16 family used. The M-14 variants (Springfield Armory mainly) had the advantage of using the 7.62 NATO round giving it better range and M-14 are semi-automatic; the problem with bringing the M-14 out of storage was that most were destroyed during the Clinton administration to save money (storage costs) so when they were being issued the stockpile was quickly depleted. The Marines wanted one per squad (the desiganted marksman) and the Army then started the same idea and then the M-14's were gone.