Why would I carry an army weapon?

The other day I went to Sainsbury (a store equivalent to kedai runcit/7-Eleven/Mydin/KK-Mart etc) someone repeatedly asked from far 'whats inside?'

I dont realize she's talking to me until I realize her eyes were fixed upon my portfolio tube, the one that architecture students always carry around. And because I was so innocent, here goes my explanation that begins with 'I'm an architecture student and there's architecture stuff inside...' and actually before I can even finish that sentence, she said 'looks like an army weapon' with a very sceptical look and she goes off (leaving me hanging for a second, and laughing for the rest of the day)

Well I never realize this tube would resemble an army weapon to someone lol. Thank God I was in good mood that day.