Don't know when it was added but GO # 1 prohibits using captured enemy weapons in lieu of issued weapons unless in case of emergency and it has always prohibited personal weapons being brought into theater.
Even the SOF guys carry issue weapons, they just have more money in their budget, and can buy what they want and issue it to their folks - yes even the guys behind the fence and from the Ranch.
Everytime a new MNF-I CG, MNC-I CG, MND CG, or BCT CDR transitions it is mandatory training for the troops just like monthly EOF training and LOLW training, so I gave this briefing ad nauseum between Jan 2006 and Feb 2007 in Iraq.
Convictions for violations of GO # 1 usually result in a quick trip to Fort Leavenworth Disciplinary Barracks, very close to where I work here in CONUS.
GO # 1 applies to all military personnel and there is another GO that applies to contractors that is different from GO # 1 in several aspects (some more limiting, some the same, some looser) - whether they follow it or not is the problem, along with very few folks that know what the rules are pertaining to contractors.
Hope this helps clear up this topic a bit....