Interesting thread as I'd never even heard of a 50 rounder, neither had James Ed who made the guns. I wonder how the 50 rounders would have fared in a Sterling L2. I would imagine that they'd perform reasonably well because the ribbed top and bottom surfaces that controlled the direction of the efficient round spring (as opposed to the not so efficient oval form spring) and reduced the friction of the rounds as they traveled down - or up - the magazine made the Sterling magazines good and reliable.
As BAR says, a fifty rounder in the field....., well, that's something else!