I have successfully installed a nunchuck 7.4v LiPo in an E&L AK.
It's a pain in the butt however.
You need a pack around 20c, 1100mAh that is LONGER than the standard buffer tube pack. Because of that, it's going to be thinner and that's the key.
You need to place a stick in the gaz tube, diagonally, and the other under the top cover angled too, with the connector and wires going over the rear stick battery.
As for you M16, you can get a 7.4v 8000mAh pack to fit without any modification (it's roughly the size of a standard large 9.6v pack). I don't recommend anything over 4500mAh as the prices just skyrocket.
For example, this fits:
http://www.hobbyking.com/hobbyking/s...o_E_Maxx_.html
But this is PLENTY enough:
http://www.hobbyking.com/hobbyking/s...Lipo_Pack.html
For your EL, maybe this can fit, but I will have to measure the ones I managed to fit to confirm (don't have the gun anymore)
http://www.hobbyking.com/hobbyking/s...SOFT_Pack.html