Re: AR-15: magazine-supported prone
The thought that it will cause malfunctions comes from rifles which have a mag catch that only keeps the mag from falling down, out of the rifle, such as the AK... the AR/M16 holds the mag in a way that doesn't allow significent movement down, up, forward, backward, left, or right...
this is why EVERYONE preached "don't let the mag touch the ground", with most weapons systems it is a bad idea, but with the AR/M16, HK 416, etc... it's perfectly acceptable and will give most people a much more stable firing position.
I have personally fired full mags both semi auto and full auto while kneeling over the rifle with a portion of my weight on the rifle.