MPs, CONUS, JAG....
I'm not a JAG(Judge Advocate General) lawyer or have a JD, but I did serve on active duty for 4 years in the US Army MPs(what was 95B now 31B).
Many changes have taken place in the DoD/US military since 2000 or so.
The gun shop chit-chat & "rumor-control" you may hear from your cousin's co-worker's barber's uncle is
not the law or DoD/DA policy.
1st off, the US Army according to official sources ceased "CONUS law & order" missions or "law enforcement" in the United States as of 2012, per DA(Dept of the US Army) regulation.
MPs or "military police" no longer conduct criminal investigations, write tickets, man gates/access control/force protection, do drug suppression(undercover operations), patrol or security, AWOL apprehesions, or traffic accident investigations.
These duties in CONUS(the "lower 48 US states") are now done by DA/DoD employees(083 police, 085 security or PMCs/contractors).
To my limited knowledge, the USACIDC or CID(Criminal Investigations Command) still conducts some criminal investigations & special missions(like PSD or security details for high ranking general officers/SECDEF, etc).
The CIDC is separate from the US Army MPs or "PMO"(Provost Marshals Office).
The Provost Marshal(normally a US Army Major, Lt Col or Colonel) is like a sheriff or police chief on a US Army post.
The DC City Council to my limited knowledge, already has new bills or district laws to exempt active duty service members from certain gun-ammunition laws. This legal/legislative action was due to a huge legal mess & civil lawsuit with DC's Metro PD over a soldier's privately owned firearms & ammunition.
The reporter from the media source; The Washington Times, Emily Miller(a pro 2A/gun ownership) wrote about the new DC area bills/court actions.
Id add that in PA, US service members can carry firearms/ammunition under certain conditions but the state laws are obscure & I highly doubt these statues or ordinances are clearly explained in the Pennsylvania area police academies or sworn LE agencies.
As stated, the DoD & service branch general officers, made many changes after the events at Fort Hood, Texas & the Pentagon(subway spree shooting).
A more recent shooting at the USMC base in Quantico VA, where a OCS candidate shot 2 other USMC students in a romantic dispute, didn't help the pro-gun/armed service member cause.
Suicides & mental health issues are at a record level in the US armed forces too. Suicides now out-number combat related deaths in the US Army.
CF