Also, I don't think that the gas reservoir MIM used to power the M870 was the same reservoir as what comes with the M870. The MIM shroud is solid. There was only room for the M1A1 barrel (they mounted the GL directly onto the M1A1 barrel's forward handle attachment), and the specialized gas reservoir which appears to be a two part system in order to fit on either side of the barrel. Basically, don't expect to take the reservoir from the grip version of the M870 and fit it in the shroud.
If you really want to have the M870 fire, probably the best choice would be to use the CA870 (the spring version of the M870). Except it has a magazine that extends a bit out of the bottom of the M870's receiver.
Even if you are able to get the M870 to fire... where will you put the battery for the M1A1 AEG? And here we are full circle again on this age old arguement.
I look forward to the day someone makes an AEG PR that has a rackable GL which fires. Until then... I'll settle for my AEG PR that's rackable.
-Alex
"The M41-A. When you absolutely, positively have to kill every mofo in the room. Accept no substitutes."