Holding the gun by the magazine itself can cause misfeeds not only in airsoft guns, but in real steel guns as well. I would not recommend this method. Specks: You use an AR-type rifle, correct? My preferred hold is on the magazine well itself, using your knee to balance.
yeah, i use an AR. I do this when kneeling, but when lying prone for a normal period of time (1 or more hours) i would rather use a rest than my hand.
For those suggesting the hand lying prone... Im a sniper, i sit for an hour or two (in the summer, to wet in the winter

) in a single spot, not a riflemen who only has to go prone for max 10 min or so. I might just pick up a bipod and carry it around, but i would rather not...