Yeah, we had a company live firing at Safti where we used the
drums. Mind you, while we indented ammo for the whole company, two
thirds of us were attached out to sports teams at the time, so the
rest of us ended up having to expend everything. Got about 400
rounds on the Ultimax and 1000 on the GPMG, and I couldn't smell
anything but Cordite for the next week. 
The drum sat more snugly in the Ultimax than the magazine did - I'm
not too worried about it dropping off. For the old top-loading
weapons like the Bren, the key design consideration was the ability
to load the weapon without having to lift it up off the bipod - a
mate of mine served in the British Army in the late 60's and used
the Bren, and he loved the ability to change mags without having to
move out of firing position. Gravity's got nothing to do with it -
if the magazine/drum isn't seated properly, it wouldn't feed
properly regardless of whether it was on top of or under the
weapon.
