I've only ever seen them feed from a box when they're mounted.
The belt being loose isn't so much of a concern during firing it, more during transport. A little context, perhaps. I live in Australia. We (our military) fight in jungle, scrub and woodland unless we get sent to desert countries that are none of our business. Loose belts and foilage don't mix.
Also, everybody knows the Elcan is the best scope out there
(Yes, even better than the SUSAT, which I love)