Never seen this modification before but thought it's pretty neat. Just got this in an email forwarded from my Dad who works in law enforncement. Here's the copied email text, it explains a little bit about how the mod works...
Images (click for full screen image)A friend of mine showed me an AR-15 which he modified. Any weapon can be modified, but I thought this modification was one to share for officer safety purposes. In picture one, it shows the AR-15 with a Magazine. Picture two shows the AR-15 without the magazine, or so it appears. The remaining pictures show that the modified magazine is actually located on top of the upper receiver and the modified magazine feeds the rounds through the top of the rifle. Which would make the weapon appear unloaded (with the exception of the round in the chamber) when the standard magazine is not inserted. I was also told that if you modify the standard magazine, you can insert the standard magazine as normal and it can then be used to hold and collect the spent shell casings. The modified magazine holds approximately 50 rounds and if modified, the standard magazine can hold and collect approximately 50 spent shell casings. Just something to look out for and be aware of. Take care and be safe.