Yup. Decades ago when I was in the 101st Airborne Division (Air Assault) up at Fort Campbell, Kentucky, we had this duty. We were on alert for a week. We had a four hour deployment time also. We were locked down at our barracks the entire week, could not go anywhere except around our battalion area. At night they would allow people to go only to the movie theater on base where they could be called out to report back in quick time. We were to go to Campbell Army Airfield and load out on Air Force transports to go to whatever city we were going to support local law enforcement in cases like this. But, we were not MPs, we were a combat infantry battalion, four companies of grunts with M-16s. And yes, we carried ammo, but the company armorer had custody of the ammo and would issue it when the time came to put us out, if they determined we might need it. We were there to back up local law enforcement wherever in the States we were needed.
Yep, July 24 '67. I was with 504th, they flew us dead of night to an empty airfield, no idea exactly where and we disembarked and sat on out butts in an empty hangar for 24 hrs, then flown back to Campbell. Weren't issued mags for the 16's, but there were ammo boxes with ready mags at the hangar door to grab if the order came to move to the ground transport outside.