I would assume feeding from a bottom mag would have the same feeding issues seen when you try feeding a 22-250 in an AR10. If the round is TOO short, it's prone to getting out of alignment before it starts to enter the chamber.
That's just an assumption though.
A work-around-fix would be to just modify a mag or build a new mag that lets the bullets ride in the very front of the magazine, perhaps with a false back, a modified follower and repositioned feed lips.
