Owen sub machine gun.

With the gravity assisted feed on the magazine, it meant the spring didn't have to be as strong,
Easier reloading of the magazine, It gets pretty hard to fully load a 20-30 round normal magazine, those last few rounds go in pretty hard.
Plus less stress on the lips of the magazine, that holds the last round in. That's one of the first parts that usually start going bad in a magazine.

Indeed I was wondering how top loading makes the difference and that does answer. The sights would have to be offset but a SMG is not the world's most accurate weapon so we can pass on that.
Also it would allow the user to lie prone much easier as well. it's good reputation was never made good as it certainly was a fair replacement for the heavy expensive Thompson. Seemed only to be built in moderate numbers.