Sorry to hijack the thread but since the original question has been answered what I don't get is why this wasn't made standard:
http://2.bp.########.com/-wB5Px2sMvOI/UpS8bdR2q6I/AAAAAAAAApE/du-mLJ-2uCA/s1600/IMAG0142_1.jpg
I mean if I had to run across no man's land and jump into an enemy trench and there was a way to increase my rifle's magazine capacity by 400% that's something I would be interested in.
And in staying on topic for the sub forum I do know they were issued to snipers, I assume becase they couldn't use stripper clips with a scope mounted. I don't really get the need as many modern sniper rifles have fixed five round magazines and snipers don't tend to shoot too fast.