I saw those folks as well at airports and large train stations, they were always in pairs and the one would carry the weapon while the other was in control of the ammunition. The idea was that it would be much less likely for two officers carry out crazed conduct with the weapon by dividing the responsibility. Knowing that maybe you could have relaxed a bit.My only experience of young lads carrying machine guns, was when I was traveling through Germany and Italy in the late 70’s when the Baader-Meinhof crisis and Red Brigade were a big threat. found that stressful.
