I've been using that tube for over a year now. I usually have it half full which is roughly 50-60 blanks at a time. I've changed my load out now and no longer use an mp5 so the tube is just loose in a zip up pouch. It rattles around and I haven't had a problem yet, so don't worry about it. As someone else said, you have to hit in the right place pretty hard for it to go off.
After the mp5 set up I stuck it in pistol mag pouch. One time I didn't put the cap back on right, bend over dropped about 30 blanks. I was playing in ucap bunker where it was pitch black, I couldn't see a thing. That's why I decided with a zip up pouch, if it happened again they'd only go in the pouch.