Equation
Lieutenant General
Good environmental awareness. Also cost saving.
Sure beats picking up empty brass casing from the ground and put it into one's K-pot(Kevlar helmet) and dump it onto a big bin for recycling after firing practice at the gun range.
The first two photos do not seem to be related. The receivers are different in shape, the magwells are quite different, and the first rifle has pic rails on all 4 sides of the handguard while the second one uses a different system to mount accessories.
Receivers are easily interchangeable, that's why it looks different. Heck I've done it to an M-16/A1 and M-16/A2 before.